Santander UK has its own UK banking licence so the usual depositer protection rules apply. However since it owns a number of institutions (as do many of the other large banking groups) the maximum level of protection (currently £85k per person or £170k for couples) is split between any deposits with any of those other institutions. This may not pose a problem for many people but for those with greater savings (sitting on the proceeds of a house sale for example) it would do if the bank were to collapse.
http://www.moneysavingexpert.com/savings/safe-savings
Having said that, the repercussions for all of us (savers or not) of a bank like Santander going belly up would be severe.