While Parler is somewhat irrelevant and the likelihood of any bank being taken down for similar reasons as Parler, it isn't completely incomprehensible.
There have been many horror stories of businesses being banned, blocked, or messed around by Google and Amazon. Their policy does not protect anyone but themselves. It is within the realms of reality for bank to be taken down by them in a matter of days.
I have my own horror story, and that's not even about the cloud, just how centralizing a service magnifies issues.
I work for near FAANG company who still sends important email information out to people. One day, many years ago, a new gmail feature landed: automatic smart tabs - and ALL of our email started landing in promotions. And those mails were send from IP addresses which were dedicated for these and only these kind of email, so we started to panic.
We started going through the correct channels for reporting this as a problem, at which point we got a "cheers, we'll get back to you in 2 weeks". At that point we were certain we were loosing money in the millions soon, so we walked over to marketing - Google PPC (pay per click) to be specific, and asked them to get hold of someone high enough at Google, we don't care how, or over what channel.
Within an hour, the change in gmail to put everything in promotions that was "noreply@" was rolled back, and our arses were saved.
If we were still in the early 2000s with countless small email servers and providers, if one of them done this, their customers would be angry at them. But since people believe Gmail is their saviour - and because nobody can get hold of anyone at google to report a problem - now it was our fault, despite the fact that we had absolutely no idea what went wrong, and there were no changes on our side.
My summary? Fuck the cloud and the centralized services; I want the small, loosely connected internet services back.
These things do happen, but I'd be willing to place money on it not happening to a major financial institution. The day AWS and friends summarily terminate the account of a major UK bank, causing people to lose the ability to access their money, would also be the day that every company in the country immediately pulls out the business continuity plan and digs into the section on dealing with having to migrate to a new provider.
I doubt the payment systems would run on a 3rd party cloud provider. But there are many other systems used by a bank, to calculate their risks, run their accounting, do their regulatory reporting, all the network drives that are used across the bank, emails, that could be killed in such a scenario. That alone is enough to threaten the stability of the banking system.
Any provider offering this kind of service is exceptionally unlikely to offboard any financial services firm without serious forethought- any outsourcing of critical processes like this would need pre-approval from the FCA and for a bank the PRA would take a hard look too.
I have little doubt that the major cloud providers would be willing to contractually bind themselves to a notice and appeals period that would suffice for “because we don’t want your business anymore” reasons.
The cross-bank correlated technical outages and general “fear of the new (15 years now, but continually being enhanced and changed)” is harder to get beyond.
Notice and appeals period isn't good enough. It takes UK banks millions to migrate to a cloud provider. Being given a notice period just means they have less than x amount of time to potentially spend equally as much as it took for them to get onto the cloud provider in the first place.
Kick a UK bank off cloud infrastructure for no good reason and you will get rung up by the PM, who will make it clear that you will reinstate them or be forever banned from government procurement.
The issue is that the Cloud provider is not UK-based. If they were to ban a bank, it's probably due to pressure from their own government (US) who could be having some problems with their client's government (UK). It's not like it is not happening right now.
There have been many horror stories of businesses being banned, blocked, or messed around by Google and Amazon. Their policy does not protect anyone but themselves. It is within the realms of reality for bank to be taken down by them in a matter of days.