Difficult one really. Generally cars tend to hold their value in Spain quite well which means second hand cars can be expensive here.
Do a search on Google for "coches segunda mano" and compare how much you can get a similar second hand car in Spain compared to a LHD in UK.
Also, remember that with the current exchange rate being so poor it might be better to buy a LHD car in the UK. I suspect there won't be that many Spanish reg cars in the UK (although I could be wrong) but if there are the exchange rate alone could make a case for that option.
When we looked at LHD cars in the UK before coming over here 4 years ago, they were actually quite expensive. There seemed to be a premium on the fact that they were LHD, which seemed very odd. We ended up buying a used car off of a friend when we got here instead.
Hope that helps.
Justin