If I can't do it myself, I use Bristows. Been going there since they opened (1980).
My issues with Bristows, and it's more my problem than theirs, is they are always booked up 2 weeks or so in advance. So if you need something doing in a couple of days, or want something quickly checked, then you're out of luck.
For example, if I went down there tomorrow and asked them if they could check the Boardman's headset/tighten stem with a torque sometime today, they would require me to book it in. Which is fair enough, but a pain to wait 1-2 weeks for a 5 min job.*
*Again, like I say, it's not their fault for being busy, and I am aware if they did this all the time they wouldn't get to the booked in jobs, but just means I have to go elsewhere. Which is a shame, as they are nearest to me.