Welcome to PPRuNe!
There are loads of airports and schools around south London... Biggin Hill is your nearest airport, but I'd suggest you visit as many schools as you can, and pick the one which your gut feeling tells you is right!
I learnt at White Waltham, and my current aircraft is based there. I chose them for a number of reasons, but the main reason was because I liked the "flying club" environment - I felt like I'd still be welcome there after I got my PPL, which wasn't a feeling I got at other places, which were more like schools than clubs. Of course everyone has different priorities, and this might or might not be a valid reason for you to pick a school - only you can answer that one.
One piece of advice when looking at school - the best time to visit is in bad weather, when most of the instructors will be on the ground. Try to go early in the day - if the weather's bad all day and not forecaset to improve, you might find that instructors leave early.
Have fun!
FFF
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