After 7 years of constant war with the UK, I have finally broken it. This was done using the Historical Plausibility mod, but it actually works with almost every mod, and even vanilla (though it is harder in vanilla).
In these 7 years of war Britain lost Ireland to rebellion, a colony to Gabu, 3 states in the British Isles to me, the Scottish Highlands became independent, Ionian islands became independent, Britain was humiliated and down to size, and were forced to repay debts 2 or 3 times. They finally broke and became England, loosing most of their cores; all in exchange for only 24 infamy on my part. Now they have no navy, a grand total of 15K troops in Australia and can only mobilize 20k more. Between that, the rebels and their grand total of 3 prestige... I think they're done.
First, the UK is big and powerful as a whole yes, but you don't actually have to fight more than one part of the UK or actually any of it's puppets. To make things better, the British Isles part that will give you the most benefit to conquer, is only lightly defended at the beginning of the game. The country that I find is best suited to this task of destroying the UK is the Netherlands.
Yes the Netherlands is small, and doesn't really have much of an army, but they do actually have one of the strongest Navies in the game. If you recall your East Indies navy along with whatever forces you can carry to the mainland, you will actually have a navy around half-2/3 the size of the British European fleet- combined with new ship building that is.
This by itself isn't enough to beat the Brits at home though. To do this you will need allies. Fortunately, the Netherlands can actually ally with both France and Prussia with almost no effort, and both will join your war 90% of the time. Additionally, the nations of Sweden and sometimes Two Sicilies and Denmark will also ally with you sometimes.
With your allies secured start justifying war-goals against the UK. Usually it will take at least 2 big wars to cripple the UK, but sometimes you get lucky with your own, your allies war-goals and rebellions.
In this case, I got a CB to free people and acquire a state. When it came time to declare war, I declared war on Belgium to conquer them, breaking the truce and taking the 2BB infamy penalty. This dragged in their allies, the UK and Denmark in this case, against the Netherlands, France, Prussia, Sweden, Greece and Two Sicilies.
What is great about this setup is that you don't actually have to lift a finger to conquer Belgium. France and Prussia will take it over in a month, causing them to sue for peace, which just leaves the UK and Denmark. I actually got lucky an got a free place in the sun CB against them and got Iceland and Greenland due to some Prussian assistance.
That just leaves the UK.
The UK at this point takes a bit of effort. Your allies are probably very stupid and got all of their fleets destroyed, but they also probably scattered the British ones. The best approach is probably to merge all your fleets into 1 giant one and hunt down all of the individual smaller British fleets. Eventually, you will destroy all of the British fleets with little effort and leave the British armies scattered on 4 continents.
At this point you just slowly occupy the British Isles and tick up war-score, hopping that you get a peace offer instead of the war-leader (if they also add a war-goal).
In my case, the European Alliance whittled down the UK and destroyed their transport capabilities. Then the US decided to declare war on the UK for... reasons, and wiped out their Canadian forces and then something strange happened.
In about 1839 or so, the Qing empire decided it wanted it some Macao, and the UK, even though it had all of the British Isles occupied, decided to join the war on the side of the Portuguese. Without proper British support, the Chinese managed to occupy all of India and even occupy half of Australia.
With 3 major wars going and 100% war exhaustion (plus another war against Gabu of all people, which they also lost) Britain eventually suffered an Irish revolt, which resulted in the complete independence of Ireland. At this point, Britain sent a peace offer to me for the Eastern counties, the Midlands and Wales, along with the independence of Scotland in the Highlands. Soon after, they surrendered to the Qing and Gabu, leaving only the war with the rest of my initial alliance.
By this point, Prussia had added a cut down to size war-goal and a humiliate one, while Greece added a free puppet for the Ionian islands. Then Britain declared bankruptcy. Seeing my opportunity to intervene again I did so, declaring war with the repay debts CB which does not have an infamy penalty for breaking truces.
Much to my surprise, Britain was actually not funding their armies at all, like 0% supplies or military funds. This leads an epic stack wipe where I destroy their 80K+ force of conscripts with a single 3000 infantry regiment. Kind of sad actually. I peace out after killing every other force I can find, which results in them surrendering to me and Prussia both.
At this point, due to the independence of Scotland and Ireland, they become England and loose a bunch of cores (only possible in some mods). They then declared bankruptcy again, which led to another pitiful war and the current situation.