jcm900 wont get you into heavy territories without loud volume and a boost, that, and they're fizzy and not very nice sounding unless you mod it to take out the transistors and opamps. However, at ear bleeding loud volumes, they kick ass. Boost optional here--> i'd recommend a RAT or a TS808
Jcm800 can get you into heavy territories but NEEDS a boost. Again RAT or ts808. Rat will get you close to deicide metalzz, ts808 along the lines of Mastodon --> mind you mastodon also has ALOT of poweramp distortion.
5150/6505 is a good choice, but you'll have 600 quid left over. Also, very very sad cleans. I'd take one if i wanted tight, raw, sterile grit, think Immortal, dark tranquillity, inflames... that fizzy grit they have. Get a bias mod done so you can run the tubes at optimal values, stock 5150's typically come with VERY cold powertubes.
fireball is another good choice, but its SUPER sensitive to pickups, so expect drastically different tones between guitars. Mid scoopy ish and has all the basic controls you need.
Powerball is like a fireball on steroids. Brewtalz brewtazls tight grind grind brewtalz. I think the best example of its tone can be found with Job for a Cowboy (they use powerballs and uberschalls --> the powerball is the more grindy one). Mind you, its hard to mic up because its mids sound scooped, is best matched with an engl cab. Super rediculously tight. Great cleans.
Savage is looser in feel to a powerball, and more versatile. It can do rock, hard rock, metal, and sweet cleans whereas the powerball is just METULLZZ and sweet cleans. Good example of this tone can be found on Dimmu Borgir's Death Cult Armageddon album.
I'd personally avoid randall's and kranks. Randall's have those hybrid poweramps which dont cut as well and dont sound as fat as a full blown tube amp. Kranks are also way overpriced for something only half decent, and you could do MUCH better for the same price.
My vote would be the savage, 6l6 version. I'm sure you can find one used within your budget, and engl users dont abuse their amps so finding a good quality used one shouldnt be hard. :) :)
hope i helped.
