I know its the one thing every guitarist dreams of, but does anyone know of any pawn shops that consistently get decent amps/old effects in? Im thinking particularly in London but I figure if anyone wants to share their secrets they'd be welcome.

There's a little shop in Newcastle (Australia, sorry) called Dinny's something something, on the main street kindof between the big guitar shop and Newcastle Station. They've had some very nice stuff pass through. The first time I went there I played a (very) nice 90's Squier tele, then a 61 or 62 SG Melody Maker through one of those little Pignose portable combos (which I fell in love with and bought). He also had an old Rick 12 string acoustic and a whole bunch of newer strats. He didn't have too much in the way of amps or effects, and the guitars were no cheaper than you'd expect, but it was a very nice atmosphere at the time.

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Fender Japan 60's Telecaster with Bigsby
Fender Japan Jaguar

Korg DT-10 > Danelectro French Toast > Proco Deucetone Rat > EHX NYC Big Muff > Ibanez FL-9 > Dunlop TS-1 > Artec ADL2 analog delay

Zoom RT-223 > Keeley Boss DD-3 (behringer FX-100 in delay loop)

Musicman 75w 210 combo