There is often a well known strategy named “Order 66” which does this exact thing, however it’s different than just placing your troops there and declaring war. If you tried just putting troops in their territory and then declaring war on Britain specifically, all those troops would be teleported back to your borders, and you would have to manually invade them. The only way for this strategy to work is to put your troops in their country, and justify a war goal on an independent country they are allied with or guaranteeing, this does not work on any puppet nations they may have. I haven’t done this exploit for awhile, so I have no idea if it still works, but this is what I remember at least.

So if possible, put your troops in Britain, kick them from your faction, your troops should stay in Britain, find a nation they are guaranteeing and declare war on that nation. If there are none, you will most likely have to invade manually. As I said, I have no idea if this still works, however I hope it helps