Cool! Definitely give me a shout when you're in town!
Flashback is good as a general new and second hand shop. They've got 3 stores now (new one in Shoreditch where theres now a cluster of 3 or 4 new shops). Their original shop in Crouch End is small but there's two other good shops near to it (Audio Gold and The Little Record Shop).
Soho used to be full of record shops but a lot have closed. Sounds of the Universe is my favourite for new music of all genres. Good second hand jazz, african and latin selections in the basement. If Music is also a very nice spot (if they decide to open that day and let you in - ring the bell in the street to get buzzed upstairs)
My number 1 shop is Eldica on Bradbury St (behind Dalston Kingsland station). Andy is a great guy and they have awesome second hand hip hop, soul, funk, uk boogie and rare rare reggae sh!t. Dam Funk and Stones Throw heads shop here which says a lot.
Also, if you're into jazz there is an amazing shop in Putney (hopefully still there). Called Soul Brother I think.
And in Walthamstow there's a spot called Alans Records which is supposed to be great, but I'm yet to check it out.
Most of those (except Soho) are in different directions out the centre. But worth the journey, plus you'll see a lot more of different bits of the city!