There won't be anyone else in the UK who has the experience that SJS have. Not just in relation to the number of hubs, but also the user feedback and the relationship with Rohloff. I don't know if they're offering wheelbuilds at the moment, if they are, they'd be my first choice despite the distance. If not SJS then I'd use another Rohloff service centre, I don't know of one in London.
What I wouldn't do, if I could in any way avoid it, is buy a hub from one supplier and have it built by another. In the unlikely event of an issue, the most common of which is a flange breaking, you want one supplier taking responsibility for getting it sorted. The last thing you'd want is the hub supplier blaming the wheel builder...
SJS weren't building wheels when I bought my Ogre hub last year, so I built it myself, using my Thorn wheel as a guide I've copied the spoke pattern (Which isn't the same as in the Rohloff manual) and the exact same tensions. I probably spent four times longer building it than any other wheel and it still makes me nervous to think about it!