In the last 50 years people in the UK turned their back on wrestling in favor of boxing(strangely enough, UK pro wrestling was huge in the 70's and 80's).
Clubs began shutting down due to lack of numbers and old knowledge was lost, now with the rise of MMA there is more interest in wrestling in the UK, but they're starting from a deficit. Due to being a niche sport in the UK there is a lack of good sparring partners to work with, no competitive wrestling competition scene, it's not in schools and coaches severely lack in knowledge compared to countries where wrestling is a strong part of the culture.
Now compare it to boxing. There is a boxing club in every town, probably two or three, a healthy amateur and pro scene, real emphasis on developing kids and great coaches.
That's why most UK based MMA fighters have great stand up yet lack wrestling.