Could I create an account with an EIN in the USA?

It would be interesting to be able to open an account with this other method of tax identification, and when it will be available to open a dollar account for users in the UK

Based on this I would say no, at least at the beginning

An EIN is the US tax id for a business. I don’t see any signs of Monzo opening business accounts in the US in the short term, so the answer is almost certainly no. As an aside, when I have opened a US business account with an EIN, I have been additionally asked for my personal SSN.