The Business FAQ has the following info:
Revolut is an FCA regulated electronic money issuer and as such fully complies with the requirements set out in the Electronic Money Regulations 2011. All client funds are held in a ring-fenced bank account.
In the unlikely event that Revolut were to become insolvent, your money will be completely safe and returned to you.
Currently your funds would not be covered by the Financial Services Compensation Scheme (FSCS). This means that if the bank that holds your funds were to become insolvent, your funds may not be guaranteed.
It would be useful / reassuring to know exactly which bank funds are ring-fenced by, and what the procedure would be to solicit a recovery, so there can be some assumed parity in safety between Revolut and a regular bank account.