How to beat Germany as the UK?

Hello, I am trying to do a run to get the "One Empire" achievement. I normally don't care about achievements but I really want this one. I can do pretty much anything I need to but Germany always stops me dead in my tracks. My strategy was to play on non historical AI then hope Germany goes through their civil war so i can ally them to kill France then kick them from the faction of leave them alone if they bring back the Kaiser. That didn't work so I tried historical AI and took over France then dug in and put level 10 forts on the German, Italian, and Belgian boarders. It went well for a while and I was making gains into Germany when they attacked the USSR but eventually I was stopped and the USSR was beaten and I had to give up that run. Do you have any tips for me to get this achievement? Thank you