I’m still driving my LHD motor I bough back from Spain in 2007. Being LHD has never been a hassle in the UK and it was easy to register at the DVLA. I think you have to deregister it at your local Traffico in Spain though.
Otherwise try sites like mobile.de to sell it – or just Ebay it but you’ll need someone with the right Spanish paperwork – NIE number etc to sell it to if it’s still on Spanish plates.
Personally I’d stick it on UK plates and see how you get on with it. If you find it a hassle I reckon there’d be a good market for the Eastern Euro folks living in the UK or people wanting to take it to France where s/hand cars cost much more than in the UK.
You will need the EU cert of conformity to register it in the UK. An Opel dealer should be able to get this for you if you don’t have it.