Mariqueue Hi, I'm also from England and I started cheer when I was 18 :) What sort of age are you? The benefit of the UK is that the majority of the teams have a significant number of adults, so you're not at a huge disadvantage if you start late (as most people in UK cheer do).
If you let me know whereabouts in the country you're based I can probably let you know the names of some gyms which hold sessions called "open gym", where you can go and train without being on a team.
I started just under a year ago, very inflexible, and now I have both front splits and I'm close to box splits. Even if you don't want to fly you should try to stretch a few times a week (make sure you warm up first!) because flexibility is really useful for tumbling and jumps as well.
If there aren't any teams you can train with nearby then you could try to join a gymnastics class to help with the tumbling, although it's quite difficult to find tumbling-only classes.
And if you can't find anywhere to help you train then plenty of teams will take people who have never cheered before :)
Good luck, it's such a great sport to be a part of, and ask if you have any more questions!