I have recently bring my Honda cb four 1972 in London and I am looking for a garage to do some repairs. I have bring the bike to a Honda garage, but the bike is apparently “too old”…
The Classic Car Club, although it’s an actual garage for storage rather than working on your bike. They’d let you do superficial stuff but I’m not sure they’d be too happy with engine disassembly or stuff like that… (they are super helpful and really nice guys though…)
However - they do use Dalston Motorcycles (I think) to do work on their own / club / customer bikes, I’m not sure if those guys work on classic bikes but worth asking?
http://www.reg-allen-london.co.uk/ Might be worth a shout , as much as a museum as a workshop. Not sure about Honda, but definitely old British Motorcycles.
Could also try ace classics in Lee/Hither Green. They’re mostly triumph specialists but may be able to help you out. They’re top blokes down there so worth a try