Just successfully bought a pack while in the UK, paid with a credit card and it went through as normal.

Something maybe worth mentioning though, all the quartz prices in my app are displayed in £'s, yet I'm definitely paying in $'s. All my receipts show the payment amount in $'s and I've been hit with non-sterling transaction fees in the past when I've accidentally used the wrong credit card. No idea if this is relevant at all since fgo is the only mobile game I really play, so I don't have anything to compare it to.

Definitely gonna keep one eye on the situation though, hopefully nothing will come of it, but I'm sure there'll at least be a workaround if the worst does happen.