Hi Monia,
Good to have you - I'm based in Norwich but I'm down in London a few times a week
My advice would be use Google to check out your local dive centre and go and have a chat to them. All of the PADI dive centres will put you through the same course to the same level, so it's really just down to who you get on with as a person.
If you're in any doubt of someones qualification, you can double check with PADI. Just call into a few dive shops and have a chat with the staff and tell them you want to start diving and see who you take a liking to!
Lots of people on this forum will be able to give you tips and answer any questions you have on dive theory.
Good luck!