Southwark College at Waterloo I believe has a good floristry department ; I've come across a few people over the last couple of years who have attended courses there (City and Guilds)
I'm pretty sure that my tutor also teaches in SW London in Clapham. Might be South Thames college - I'll ask her on Saturday. She's fantastic, so if you got onto one of the courses she teaches you'd enjoy it, I'm sure
