Sending a large amount of money from Spain

My in laws have just done this - whilst this is not advice it does look like paying the CGT in Spain has saved them enough to make it pay to have an accountant back here handle their returns for the year the transaction occurred.

Things, in my ever so humble opinion, they might have researched prior to shifting the proceeds back to the UK...

SEPA - try not to let the bank into which the proceeds are deposited hit you with excessive FX charges, SEPA is there to be low cost and quick.

Using a specialist FX house, one with a Spanish presence into which you can have the proceeds transferred prior to them doing the FX internally and them sending the money on to your chosen UK account. Your Spanish bank may claim they can't transfer your proceeds to such an organisation, I suspect this is a ploy to keep you within their more expensive FX system (I can't state categorically that this is why they behaved this way with my in laws, could be my suspicious mind going into overdrive).

There are quite a few FX specialists around, HiFX will pop up in a Google search with an Independent paid content article but it explains things clearly. Hargreaves Lansdown are plugging their offering a fair bit at the moment.

Doing a bit of research could save you a good few quid so, if you can, take your time.