I'm sure there are lots of other great shops in the London area, but these are some of the ones I have taken notice of....
Frith St (obviously)
Frith Street Tattoo | World famous Frith street tattoo, custom tattoos situated in the heart of Soho, London. Walk ins welcome!
The Family Business
The Family Business Tattoo Shop
Ian Flower's shop
Welcome
Into You
London - Into You
Jolie Rouge
http://www.facebook.com/jolierougetattoo
Jayne Doe
Jayne Doe
Alex Reinke is also in London, but I have no idea what sort of waitinglist he operates with, or if he even does one shot tattoos. You can always email or call him though
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I pm'ed you about Norway ;)