How about something like what’s on here?
http://www.kgbcameras.co.uk/cameras/wireless-webcam.htm#Wireless%202.4GHz%20USB%202.0%20receivers.%20%20Quick%20summary:
Maplin do a ‘Wireless Webcam’ but it’s only got a 1-2m range, so useless really.
KGB cams stuff is prolly more what yer looking for. You can get standard video (RGB+Composite) type wireless cams, but then you’d need a digital video encoder or some sort of video capture card so would end up costing more I’d say. If you have a DV camcorder, you might be able to use that as a bridge between cam system and computer. But the main obstacle is the USB interface bit really. Cams themselves are dirt cheap, like £20 or something.
You could spend a fortune on pro-quality gear, but maybe a bit overkill. An infra-red system might be an idea for dark places.