wembley guitar centre is pretty good- engl, laney, mesa, peavey, hughes and kettner, marshall head to head. they have a sound proof room too, so you can crank (might want earplugs for those 100 watt heads, though).
oxford guitar gallery is extremely good too, loads of boutique stuff.
i guess chandler's is worth a travel to too, haven't been there myself.
guitar guitar isn't bad either (edinburgh/glasgow/newcastle)...
probably loads more, that's what i know of offhand.
as for the brand, at those types of prices it's very much personal preference... and you'd need to try something like an se versus two amps at half the price (but aimed at the different ends of the spectrum you want) to see which you prefer...
i'd have thought 2 £1500 amps would do better than one amp aimed at "it all", just on the principle of what makes a good metal tone generally makes a cr@p vintage tone and vice versa, there's going to be some compromise... depends on how much room you have, of course... EDIT: and obviously if something just happens to do the exact tone you want at £3000, lol.
but £1500 should buy you pretty good kit in anyone's book, you're already in cornford etc. british/european-built boutique territory. just maybe not USA-built boutique territory.