I am travelling to London this week and only signed up to Revolut a couple of days ago so it’s too soon for me to get a card delivered. Is there anyway to withdraw from an ATM in London without a physical card?
None that I can think of 
You could find another Revolut user, and send GBP to them, which they can withdraw for you. Failing that, ask someone if you can send GBP to their UK bank account, and then ask them to withdraw it. Transfers between UK banks are usually in real time, although I’ve never tried this with Revolut.