>Revolut can just refuse to reply and that's that. They also told me as much, they said I have to file a formal complaint with them first.

Revolut can't refuse to reply. They are literally advising you on the first step to file your complaint in this procedure, as prescribed by the Financial Ombudsman Service. It is there to exhaust your query, in order to either resolve the matter amicably or to arrive at some satisfactory conclusion and more importantly it exists to filter out any frivolous claims.