You've not read the thread properly. With some human help the UK can actually do well. On a purely AI vs AI basis, however, France will nearly always dominate the UK, unless some freak accident involving a Gutter Crown Germany.
Nearly everyone is reporting the same problems. The UK will land only a few stacks in France at a time, it'll keep the rest in Ireland, the Royal Navy will get destroyed and fail to rebuild and French will invade the mainland with ease. Nearly every war I've seen with the UK starts off with the UK doing well, with several red stripes often appearing within France not too long after the start of the war. Two years later and France will have cleaned up with its behemoth of a mobilised army, the UK's navy will have collapsed and 60,000 troops will be stuck in Ireland twiddling their thumbs.
Okay, your point. Hands off, UK is likley to be outpaced by france, this will even happen if they aren't confronted directly. I would say, the human interaction with the UK feels okay. As mentioned they actually do sth when your are allied with them. If you are a small country, they are a threat. When you go against them as a powerful country, that is the weakest point. With a right set up war, you'll beat them. But thats the case with every other country aswell. Its only a bit of a diffrence because they should be the dominating power and they should be able to keep the status due to theyr starting postition.
I know these "historical..." and "important for gameplay..." Arguments are disliked by some people, but would it be lots of fun if every early confrontaition with the UK, lets say before 1900, would end in being destroyed by a rageing giant whos bringing all the stuff he has at you? For me I would say no.
Further it wont feel proper if they would do so. Manageing a colonial world empire doesnt allow to concentrate almost all of your power on one point. You cant show large parts of your lands blank and you dont have capacity to ship one million men from india to europe in two weeks (in 1850 at last). Okay to sit on the island and do nothing, while continental europe is burning looks odd, but thats what they did most of the time. You can for sure hold against that things wouldve gone diffrent if some country had destroyed the european balace of power by hard (like they often do as human controlled in the game).
If you play a hands off game, its likely they will submerge slow but steady or within a blast of one or two great war defeats. This is not was historical happend, but with the time of 1900 - 1920 coming, the power shifted westwards to the USA. So again, it doesent feel very wrong. But honestly I don't want to watch a hands of game with "correct" results, I wanna play! I wanna crush the others! I want those to win who didn't in reality! Thats my core motivation to play those great paradox games.
I totally understand that people dont like the UK behaviour, it certainly would be even more fun if they had a higher wage in oversea wars. But I don't think they are totally broken or an usless ally. Its okay for most situations and very enjoyable if it comes down to my opinion.