In my experience, Boots or Jessops on the High Street are random. Boots in Bromley is great, Jessops not bad. Jessops in Hastings, however is a nightmare.
Kodak UK *will* scratch your negs. Every time without fail. If you get unlucky, they'll also play with it in their sand-pits and dust-boxes, which they reserve for special customers like me.
Most of the London pro-labs are good, assuming you don't get a college / work experience youth. It's common for them to string all the negs together and dev them on a giant drum, which some people object to.
My best experience with photography has undoubtably been Russia. Stern looking bespectacled staff who really knew their stuff, wore latex and cotton gloves and forced you to go through your prints and take out the ones which you weren't absolutely completely happy with. They would then hum and har and try again on some of them until you were happy. This was both in the big Nikon lab on Arbad-strade(phonetic nonsense spelling) and in a normal small shop out in the projects.
Whilst we're on the international thing. Bermuda. The most unhelpful belligerant photo lab in the world - the Kodak lab in the Phoenix Mall. Charged me $150 for one set of prints!!
The best advice has been mentioned already - get to know the lab staff - preferably take them out for a beer and say how wonderful they are. This gets you the best treatment in the world. They'll also reprint my ill-judged cross-processing experiments in black and white for free (as I required yesterday!).
R