I am returning to the UK and would like to try and part exchange me Spanish reg car for a UK reg one.
Does anyone know how what and where this can be done

Yes I would think if a dealer has a UK reg car he would be only too pleased to offload because it is so much hassle for anyone here to get it onto Spanish plates - if you are in this area (Torrevieja) try Global cars

I am in Malaga

OK rules out Global Cars but the same logic applies - dealers always want to get rid of UK cars fast - ask around..

There are many Brits who want to get rid of a UK reg car, instead of re registering it so you can pick them up for a song, just make sure it has valid MOT, tax and has not been registered as permanent export.