Plus Valia,
is a tax on the increased value of the land upon which the dwelling is built (also applies to apartmentsas I believe a percentage).
It is a tax which the seller is legally obliged to pay, but it has been the practice for many years to get the buyer to agree to do so.
What can they do in Spain to recover the Debt ?
Well, even if the buyer has to pay, they can go to the court in UK and claim against your assets there.
Sorry, do the right thing, pay up.