I always use the one in Dukes Mews, Muswell Hill tel: 0208 883 1065. They came recommend to me by my mother-in-law who's an artist and gets all her work framed there. Their work is good quality and they make the frames on site (not sent away like some framers do) so they can often get the pic framed within a day. Plus I've found their prices extremely reasonable - especially when compared to the cost of buying a frame from say, Ikea and buying a mount separately. I've been really happy with them.