Having worked in banking for a number of years, id say that if anyone loses a penny from their balances held at Santander or any other bank operating in the UK, I will eat my hat. Systemic risk means the government will supplement any bank that needs refunds to repay in full the banks liabilities to its customers.
The FSCS is more of a political tool in creating confidence within the sector, when Bradford and Bingley went pop, I think the FSCS had about £2bn in it and the amount paid out of £14bn (i cant remember the figures off the top of my head, they are ballpark).
Although shares, id be concerned, unless you are going short.