I think it kind of depends if you have a good little runner and no great use for it in the UK and would not get much for it if you sold it
and then your budget to buy if you do sell in UK
and loading it up with "stuff" and driving over could be a good idea
it's not feasible going back to UK for MOT and too risky these days to keep it on UK plates (how times have changed!)
Putting it on Spanish plates will cost you approx Eur500 on a DIY basis
Downside is there is no great resale market here for RHD cars
Sometimes you see people on here looking for a swap?