For London although you don't say which part of London you are in, these are the two reputable BMW specialist that I know off:
https://www.gilbertsforbmws.co.uk
https://www.birdsauto.com
I've not used Gilberts but I have spoken to the chap on the phone prior to buying my E89, he was very helpful and knowledgeable and has many good reviews on the internet.
Birds used to look after my old E30 325i and did the Hartge conversion on it. They are superb and again extremely knowledgeable about BMWs but a little pricey.