You can all but rule out carbon for £500, that's not much in road wheel terms.
If you're not actually racing don't get tubs, they are a faff. There aren't too many aero offerings around that price either, as they tend to weigh about 5kg!
Ksyriums are often the default recommendation. I personally prefer Bontrager, Race X Lites are great wheels.
Edit: oh yeah, forgot Planet X, Pro Carbon 50s are good value, but tubs… Also bear in mind carbon rims are pretty sketchy in the wet, they really would have to be a 'dry weather only' wheelset, which ideally means changing pads whenever you change wheels.