I want to attack Great Britain. How can I defeat the British army? I want to become the King as well

Hmmm, not totally convinced...but it's still a pretty good answer. Have you read any of the War Nerd's stuff? You might like it. Oh and if you don't mind a little Sci-Fi in your historical fiction, you should check out Connie Willis' The Doomsday Book. Finished it last week, really enjoyed it.

make a tea party. get elected cause British loves them teas. throw tables and become king. of course there is a robin hood. marry Marian. robin gets mad cause potato. invite robin to tea party (this time an actual party). he accepts cause British loves them teas. poison robin's tea with them bad sensual teas. robin flips table. u grab teas cause that's ur job as king of british pll. robin drinks tea but first king must drink. after that you bring them cakes cause tea and cake is awesome and you are AWESOME king. you feel a bit dizzy in the head. but that's not the sensual tea cause u r immune to that shit. after going home u see Marian is cheating on u with that monk and you don't even know his name. feel bad. divorce Marian. later that day robin marries Marian. at the same day Marian cheats on robin with that father monk that I still don't know his name. robin gets mad. robin kills you.

so you can't be king of uk unless you are gay and marry robin and forge an unbreakable ally and f@@k that Marian and that monk btw.

I would block every trade routes and wait until they wither away from the lack of tea, and when they are no more than husks of their former selves I would appear as a saviour, The TeaBringer.