> Makes me wonder why an EU bank is keeping data outside the EU (from my understanding, only the London data center is down)?

Let me tell you a story about an idiot politician who woke up one day and thought Brexit would be a fantastic idea, and then the stupid dim-witted "ultra-hard brexit" manner in which it was implemented by a certain Mr Johnson.

The same reason UK people can't open EU bank accounts is the same reason EU banks have separate operating companies and infrastructure in the UK.

Given they operate in a heavily-regulated environment it almost certainly made life easier for them to keep UK stuff in the UK because they had no idea how (or if) the regulatory environment would ever progress beyond ultra-hard brexit.

Maybe they mix it: serving fonts/pictures/etc over scalable cloud providers and hosting confidential data themself.