> Among people I talk with, and whose bank cards I have seen, in and around London, Monzo is far more popular than Revolut.
This has also been my observation. Multi-currency support is something I would like to see come to Monzo, although I'm not sure what regulatory constraints this would apply; my understanding is that most conventional multi-currency accounts (e.g. CitiGold) actually involve multiple accounts in several countries with the parent international bank guaranteeing free transfers between the individual accounts at market rates.
Revolut does have the advantage that it lets users maintain balances in multiple currencies at the same time.