Buster -
As it's a UK infrastructure project, which will earn its return in £££, I would expect it to be financed in the UK. There are financial instruments available (such as securitisation) that will guarantee the finance repayments out of the revenues earned. I would expect the debt to remain with the airport itself rather than its Spanish owners.
Of course, if the euro meltdown happens before finance is in place and being drawn-down, all bets will be off.