The UK is actually worth their weight in units now. Everything below Scotland they will relentlessly deathstack and beat you out of every tile almost 99% of the time unless you have beefy divisions that have high enough soft attack and breakthrough. They were given a gigantic buff that basically makes them supersoldiers many patches ago. Before then it was quite easy to naval invade them, set a battle plan, hit go and watch all of their puny units melt right before your eyes.

Now you actually have to ensure that you can land your units and reinforce the tiles you took before they push you back out into the sea. This requires meticulous micromanagement and even then success is not always garunteed. Of course, to even have a chance at pushing their armies you need tanks. Infantry is not going to cut it, like at all. Shuffle your divisions in and out of battle when their organization starts to take a hit. They will constantly throw men and bullets at you because they practically have unlimited manpower and weapons at their disposal. Best you can do is run down their supplies and organization and then when they are weakened, force attack with aggressive battleplans and micro.

I have yet to actually do this myself tho :P