Loads of great amateur gyms in London:

Repton and West Ham are probably the most successful historically - not sure if these take on novices though.

Here a some of the larger / most popular clubs in the the area:

Lynn AC, Earlsfield ABC, Dale Youth, Finchley ABC, Dagenham Police and Community, Fitzroy Lodge, Islington, Hooks ABC, Hayes ABC, All Star ABC.

Plus loads more - there must be 50 + clubs in London. Check the club finder tool on the ABA Website.