Toureman seems to be assuming that you're in New Zealand! ;-)
However, the substance of his answer is still correct. The Road Vehicle Lighting Regulations 1989 state that you can only use white lights:
http://www.opsi.gov.uk/SI/si1989/Uksi_19891796 _en_20.htm#nsch15requirementrelatingobligatory rearregistrationplatelamp
Further, I suspect that examination of the Road Vehicles (Construction and Use) Regulations 1986 will show that
(i) there are rules about the positioning of rear registration plates ; and
(ii) moving the rear registration plate (even if it's to a position permitted under the regulations) removes 'type approval' from your vehicle. If so, it's illegal to drive the vehicle until it's been taken to a testing centre to seek 'single vehicle approval'.
Even if you can get 'single vehicle approval' (which is expensive and only possible if the registration plate is in a position approved by the 1986 regulations), you'll still need to advise your insurers, as moving the registration plate counts as a modification to the vehicle. This will almost certainly result in an increase in your premium.
Chris