My colleague Simon is a national level swimmer and coach. He's just moved to Clapham but would come this way I'm sure and go to a pool of your choice. I'll ask him next week.

Cannot recommend Sam at http://www.swimstudiolondon.com/ more highly. You make rapid progress in 1-2-1 sessions on technique. Then you can join clubs, group swims and things like SwimTrek to put in the distance hours when you feel ready.
Tell him Darika said to get in touch if you decide to call him!

Spencer Swimming club has sessions at Dulwich College, Crystal Palace, Ernest Bevin and Putney and would also be able to help with this. Again you need to join but you can have up to 3 trial swims to see if it's what you're looking for

http://spencerswimteam.com/

pm me if you'd like more info.

Swim trek are good for long distance swimming - they have sessions at tooting lido

https://www.swimtrek.com/swimtrek-coaching

The Clapham Chasers triathlon club has several coached sessions per week with specialist instructors who are excellent.
You need to join the club (but it is only £30 per year) and then the sessions are I think £5 each. The coaches also do day workshops for technique at an additional charge, and give you 'homework' to help you train towards your goal.

More info on their website:
http://www.claphamchasers.co.uk/swim

Hello - I am a relatively good swimmer but my goal for the summer is to get into long-distance swimming, thus need to learn the techniques (no triathlons in my future - just for fun.) Do you have any suggestions for swimming instructors for this purpose?