Tapping a left hand thread in a smaller right hand thread

[Charlesthecab]

Looking for someone in or near London who can tap a LH thread in a smaller RH threaded control bar for me

[GreenV8S]

If the minor diameter of the new thread is greater than the major diameter of the old one then any local engineering shop would drill and retap it for you in a few minutes. If it isn't, I expect you'll struggle to find anyone reputable willing to touch it.