ngeshas:the UK Government that will step in to support a foreign bank.
I am guessing that your money is with Santander UK Plc which is a UK bank covered by the FSCS. Other parts of the group, like Santander Private Banking, Jersey will be covered by whatever schemes are in place on Jersey or wherever they are registered.
Santander has never been the most prudent bank and they would be low down on my list of banks to do business with, but any money in mainstream accounts will be as safe there as with any other bank with overseas interests.