I'm looking for a guitar with a tremelo, preferably double locking if thats a good idea, for the £500 price range, the guitar will be put in drop c, and will be used for metal. any suggestions?
Schecter are great if you don't mind big necks. Some of them feature Official Floyd Roses, and their liscenced ones are good, from what I've heard. Jackson are also worth checking out. Their pro series have liscenced Floyds, but they're really good. You can trust these two for a good trem in that price range.

ESP are also good quality, especially for metal, but make sure you get a good one. Anything with a model number below 400 probably has a crap tremolo. Look for higher-end ones.

Ibanez guitars in this price range often feature the Edge 3 tremolo, which is the achilles heel of these otherwise great guitars. They're made of some cheap metal (I think it's some zinc alloy). They work fine for about 6 months, but then the knife edges wear out and the screws might strip, and it never stays in tune after that.

Also, adjusting it for drop C is gonna be a pain. Get a pro to do it for you the first time, and use the same string guage and brand every time after that.
i already have a schecter tempest, i like the neck on it but would prefer something thinner, can you point me in the direction of some anyway please mate?
Ibanez S series guitars, in your budget you should be able to afford the s320, s470, and maybe the s520.