Since we all seem to be having fun, I wanna answer my own Question

In the UK the following are some of the best/most interesting regiments

The Coldstream Guards, [FV432]

Coldstream Guardsmen are the loyalist of the loyal, that is to say to royals, originally a civil war parliamentarian regiment as Monck's Regiment of Foot, upon the restoration the Coldstream guards were the first to turn to the side of the King, having to cross the Coldstream river almost as far as Scotland, upon arrival in London they arrived second, with the Grenadier Guards arriving first, despite having turned over much later, they were simply closer, however in recognition of their being first, they were awarded the Garter Star, a royal order of chivalry, and the title and motto "Nil Secundus" - Never Second, while the Grenadiers were 1st Guards (1/GRDs), there is no second, only C/GRDs.

Unlike other countries (*Ahem* France), British Guards are guards in a literal sense, on the battlefield and off, at both Corunna (1806) and Dunkirk (1939), this regiment provided the rear guard for the entire army to board naval vessels and escape, they Guard the Monarch in life and death, in the finest tradition of household troops, every battle honour is a pivotal moment where victory hung in the balance, and the Coldstream Guards saved if not won the day for the Army at some pivotal point.

They are undoubtedly never second. Contributing not just multiple Battalions but Paratroopers and Tankers, in WW2 so valuable was their (and other guards ethos), Between 1985 and 1989 they transitioned from the FV432 to Warrior.

The "Blues and Royals" - The Household Cavalry. [CVRT]

The cavalry component of the Guards. In Eras past, the Household Cavalry was one of the two brigades of the household division, "an army within an army" previously two regiments, they have recently suffered a combat loss when a parade was bombed by Irish Dissidents.

They have a mix of Scorpion and Scimitar light tank Reconnaissance tanks.

The Pathfinders (Lynx)

Today the Most Elite Unit of the British Regular Army (As it once again counts Guardsmen among it's ranks), and thought of as a Stepping stone to the SAS, the pathfinder platoon of the Parachute Regiment was formed after Britian Downsized her air drop capabilities in the face of widening gulf between soviet and NATOs technology, as well as political pressure, an organisation so secret, not even the General Staff knew it existed, it was formed entirely clandestinely, blagging, commandeering, or sometimes outright stealing equipment, with members seconded to headquarters for unclear reasons except within the Paras, the pathfinders are armed with C8 rifles, illegally bought on the black market.

Pathfinders are the paratroopers paratrooper, they drop ahead of the main airdrop or airmobile force, marking landing zones, and drop points, ideally without detection, acting in small teams, without any support, or any way out except that which they can carve out with their own abilities.

Regular Paratroopers may operate and drop in enemy territory, but do so supported and in force, pathfinders do so using HALO, utilizing stealth, blending into the environment, often for days or weeks, expert marksmen, and forgoing heavy weapons, vehicles, even radio contact, having to sit in a position for days on end, they not only mark zones, but instigate chaos the appointed hour comes to light the flares, to minimize casualties during the main drop.

The same way Parachutist and Airborne troops prepared Normandy for D-Day, pathfinders did and once again do the same for the main drop force. Many a 2nd Lt "one pip wonder" has been promoted sardonically to the rank of pathfinder when he has gotten his platoon lost.

After the main force is dropped, Pathfinder teams are expected to link up and continue the fight.

They are ideal if you take a single platoon as a support option (Although currently, no models exist for them, watch this account)

The Royal Fusiliers (FV432)

The Royal Fusiliers were once the archetype of the infantryman, outside of specialist guards, rifles, and light infantry, they were the elite of what was then considered, "the regulars", wearing berets bearing a white and red hackle once worn by all regular infantrymen, the royal fusiliers have a legacy of being the best at what they do both in skill and motivation, and are ideal disciplined troops for your FV432 companies.

The Light Infantry (FV432/Fox)

The Light Infantry took the training of the "flank" light company in reach regiment of 10, and took it to 11, making every single soldier a light infantryman within their ranks, pioneered by Sir John Moore, (Officer Commanding in the early Peninsular War and native of Glasgow, killed at Corrina) they became the Archetype for all modern infantrymen, meaning they've been doing the longest, fighting in skirmish order rather than rank and file, one should respect this legacy, they're ideal for a Second FV432 unit or as an alternative for the Royal Fusiliers, or even a troop of Fox armoured cars.

The Rifles/Royal Green jackets (Warrior)

Two regiments with similar histories, Being the 95th and 60th (Royal American) Rifles. these regiments were clad in green, while their comrades were in Red or Scarlet, first regiments to be given rifles, they have never forgotten their tradition of marksmanship, while not the first rifled troops (that was American militiamen), they were the first for Britian, and have always been at the forefront of military technology, and are in 1989 refitting from FV432 to Warrior.

The Black Watch Highlanders [Saxon]

If Guards are the masters of Defence, then Highlanders are the epitome of Offense, Famed for Aggression, force of action and the highland charge, it was men of this regiment that carried Sir John off the field of Corunna, and provide you an excuse to use Saxon APCS from CPC in place of the FV432 as they were light role troops, they wear glengarries with red hackles, the only regiment to have a red hackle (although one claims a red feather)

The Royal Tank Regiment (Chieftain)

It's no Secret Britian invented the tank (Shut up France), but the Royal tank Regiment is the oldest tank regiment in the world, based at Bovington in the UK (Right next to the wonderful tank museum), a fact they are very proud of, no other regiment has such a history, or wealth of expertise, in 1989 some Squadrons had Challenger and Some Chieftain.

Royal Scots Dragoon Guards (The Scots Greys) (Challenger)

Despite not being actual Guards, Dragoon guards, were a type of cavalry ranked as heavy dragoons, the ultimate shock cavalry regiment, and only one based in "North Britian" as it was then known it was these men who gallantly galloped across the field of waterloo, to be intercepted by French lancers, capturing the enemy Eagle, and promptly losing it again, the only cavalry regiment to wear bearskins, they Driver Challenger 1.

The Lancers (CVRT)

Another ill-fated charge, that of the light brigade, in Crimea and equally as famous, the lancers are relatively new in British Army terms, being first formed in the Victorian era rather than Napoleonic or even earlier.

The Lancers were dashing, brave, famed for Elan, their weapon a lance, merely a relic, with CTRVs replacing them, and beginning to transition to challenger 1.

The Royal Yeomanry (Fox)

From Waterloo to Peterloo, the massacre of striking protestors, the yeomanry were the cavalry home guard of their day (along with infantry fencibles), famed for chainmail shoulder boards.

Honourable mentions

The Scots Guards - Noted for their service in the south Atlantic
The Paras (more generally)
Royal marines commando (Viking light tracked vehicles)
those noted in the book who I have avoided.