If it were me.... (and I have a revolut card).
1. Ensure the seller has a revolut account
2. Add the sellers contact to my phone (Revolut then recognises that they have an account, using the phone contact as the link)
3. Transfer the amount of the purchase into my Revolut € account, from my normal bank account ( I only use Revolut for odd purchases, have an Irish bank account for day to day).
4. Transfer the amount of the purchase, now in my € Revolut account into my £ Revolut account. (all done in minutes on your phone). Do this during a weekday when the rates are better. You can have different currency accounts in Revolut and transfer between them.
5. When happy with the purchase, transfer the £ amount instantaneously into the sellers Revolut account.
4.a It might be an idea to transfer £1 into the sellers account when it is set up, just to ensure everything will work smoothly when you are over there, which is not the time for things to go pear shaped.
I have no idea what currencyfair is, but it is not needed as everything is done in Revolut. You will have to do a bit of reading up on Revolut if you are not familiar with it.