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