I think were the money is actually being held is fairly irrelevant:
You are depositing your money with Revolut Ltd. (If you put the EUR IBAN into any IBAN checker it will also tell you as much) and therefore any claim would be against Revolut Ltd, which is a UK company, regulated by UK authorities and under UK law (the last bit is also in their T&C).
That being said: Revolut is not FSCS protected (as far as I know) and therefore you have fairly little protection anyway. But I do not think that where Revolut holds your money has any bearing on the protection you have (or don’t have).
Finally, just a minute ago I received the folloing information through the Revolut chat:
IBANs are provided by the central bank of Lithuania whilst their money is held in a U.K. Account (sic) through Lloyds. Revolut has an e-money license and is regulated by the FCA
So, this confirms that money is held in the UK, but not FSCS protected. Instead “Electronic Money Institutions” must safeguard any customer money deposited with them by holding them in ring fenced accounts with “actual” banks, or have insurance in place in order to cover the customers’ deposits in case of insolvency. (I believe Revolut has chosen the first option.)