How to credit Euro cash/notes onto my Revolut card?

Does anyone know the best/cheapest way to get Euro cash / banknotes credited to a Revolut card?
The best I’ve found is Barclays who offer a Euro account which provides a counter service and so cash can be paid into this account at any (?) of their branches. The downsides of this are that they don’t provide a debit card and it costs £5 flat rate (?) to transfer funds to a third party account - which presumably includes the Revolut account.
Does anyone know of a better solution?

When I last looked into this, the best way was indeed Barclays (with the fee you list) or, if you qualify for their Gold account and live in London (the only place they have UK branches) Citibank (free SEPA transfers, and you can pay in Euro and USD cash at their branches near St Paul’s and elsewhere). But you have to maintain a ridiculously large balance in Citibank to avoid large fees on the Citibank account. I have a Citi Plus account which is not currently open to new customers who don’t already have a savings or foreign-currency account with Citi, for which the balance requirements are much, much, much more reasonable.

If it’s only for a one-off transaction, you really might be better off just changing the notes to pounds at the best bureau de change rate for buyback on travelmoneymax.com and then credit the balance to Revolut in Sterling.