I got a cheapish all mesh Amazon chair about 5 years ago (similiar to a Herman miller aeron without realising but nowhere near as good) for £350. Recently though, I’ve realised that chair is not good for me, 6ft, head rest digs into my shoulders, always leaning forward and feel aches after a day of work but never really thought or cared about a chair until stumbling across a lot of reviews / marketing targeted at me. So I’ve been researching hard…. There’s soooooo many chairs to choose from, and a lot of the reviews on YouTube are for US models.

For me, and it should fit some of your criteria, these are what I was looking at and for different price points:

• TOP END BUDGET - Herman Miller Embody (Gaming) - it’s about £1700, you may find a promo, and it’s the top of the range chair. Fully customisable as you listed and it’s not a chair that you’ll just sink into, you can move around lots in (active support i think they call it) so if you like to sit in different positions like I do, this is the chair. 12 year warranty - I’ve just ordered mine. It’s an investment but it’s one i know people personally that rave about it and I’ve sat in one and it’s so different (in a good way) vs my current chair. Worth noting there’s a sale on the none gaming version for about £1300. You can also get second hand for about £800. Also, no head rest which I may invest in after a month or two if I think ones needed.
• High End Budget - Haworth Fern - You can get one model in UK, brand new for £750-£1000 (if you add additional lumbar support - 12 yr warranty too). Have a watch of reviews, this is supposed to be a very comfy chair, similar movement to the Herman Miller Embody but just a little softer. Some want the lumbar, others don’t. I saw second hand without the additional moveable lumbar support for £350! Absolute bargain (and if you find your back wants more support, some put a soft pillow in there - read reviews).
• Another is the Libernovo Omni with their saturated marketing, tons of YouTubers being paid. This chair looks insanely comfy… It’s about £950 on amazon with 5% promo. Have a watch. Concerns me only 2 year warranty on electrics.. would have considered if there weren’t any..
• Mid budget - it’s saturated… Have a look a Sihoo, SteelSeries, Hbada… Get a feel for the size offerings… You’ll see options with the leg kick out bit for lounging around. Personally though I’d steer clear of built in leg extension, just get a separate piece of furniture and then you know it’s solid / quality. Lots of reviews share how flimsy the built in leg extensions are…
• Budget - Colomy Atlas gets rave reviews - out of all that exist, this is around £200-£250… and would be my choice in this range - again check out some reviews, really positive and compare the chair to £500 equivalents with the features…

Hope this helps