It depends on what you want doing. Some people to try in the first instance might be
J Roberts & Son as you are in London
Another might be Ronald Wharton in Sussex
If its anything to do with target or foxing rifles try Norman Clark in Rugby
Then you have numerous custom bolt rifle makers such as Pete Walker, Callum Ferguson and Steve Bowers
If its barrel threading or screw cutting for a moderator, alot of gun shops use Brock & Norris
Hope that helps...