CF:
This is a minefield.
Show me any FTO in the world and I'll find you two people, one who won't hear a word said against it, the other who hates it like poison.
You'll get a subjective judgment from anyone you ask. Flying schools are like shoes. One size doesn't fit all.
In the area you're looking at, there are a number of schools. Why not visit them all? You could have fun doing it. They'll welcome you - it's a bad school that doesn't try to impress a potential customer.
It's worth going further afield, too. I think there are helicopter schools at High Wycombe, White Waltham, Denham, possibly Stapleford. Other posters here could give you a better steer.
Your first impressions count for quite a lot. If the place is clean and friendly, that's good. You're going to have to feel comfortable hanging out there, for quite a long time.
Talk to the staff, talk to the instructors. You'll often get good reports from students, because they have nothing to compare their school with. In my humble opinion, it's better to have some old greybeards around - if all the instructors look like they were recently weaned, that would put me off.
Check out how many helicopters they have, or have access to. Lots of students lose out because if a school only has two machines and one goes tech, you're jiggered.
See how long they've been in business. Bad schools don't last.
I've seen lots of people go into an FTO as if they were joining a family. You're not - you're laying out a lot of money for a professional service. Make it clear you're sizing them up, be cynical and don't accept vague answers.
There are many more good schools than bad.
You've made a brilliant decision already, in deciding to go for it. Good luck with the rest.