Another interesting question. I can only talk about the bits I know about (I’m sure there are many other issues that would crop up that I can’t think of).
From a purely technical perspective, we wouldn’t have any issues with billions. Your money is just a number in a database, and our database can handle some very big numbers.
Getting your money out again at any reasonable speed (£100ks to millions a day) would be a pain. The FPS network has a £250k per transaction limit, and then there’s the “Net Sender Cap”. I won’t do into details, but basically there’s a limit to the total amount money Monzo can send via FPS in one day. This isn’t an issue 99.99999% of the time (even when dealing with million pound transfers), but would prove a real issue if you wanted to take your billions out.
We could send that amount of money using something like CHAPs, but even that isn’t super simple. We would need to work with another bank (Natwest) to make that happen. Again moving large sums (millions) is fine, but billions is a bit mad.
Then there’s the issue that if you had billions with Monzo, it would probably be more money than all of our other customer deposits put together. Which creates all kinds of conflict-of-interest problems. I don’t think the PRA would be happy that a single customer could potential cause serious capital or liquidity problems for retail bank on a whim (this is why most bank account have a limit on how much you can deposit).
We probably wouldn’t. Taking on such a customer would probably require us to give up normal retail banking, and we didn’t build Monzo just to serve the ultra rich.
Ultimately this all boils down to scale, the difference between a millionaire and a billionaire, is about a billion. It just such an insanely large number that lots of things just break.