Re: Anyone know where to go in England to find great deals on Marshalls?
Sorry, I hadn't realised you were looking for used or vintage. That's a bit more tricky as you can't predict what anyone will have in stock. You need to be prepared to visit a lot of shops! If I was looking for an older Marshall I would probably look in the Guitarist classifieds and on Ebay.co.uk first. There are plenty of guitar shops around London and the London area, so if you could get a list of them (again perhaps from Guitarist magazine) you could visit plenty in a three-day trip. It's worth looking around Denmark Street, but have some idea of UK prices beforehand, so that you can haggle if necessary.
As regards having friends look for you, I guess it depends how much they know about Marshall amps!