I must confirm that the simplest is to convert GBP to EUR first, then send the funds (in EUR) to Paymium EUR account. That's a SEPA wire transfer, free of charge. You only pay a currency conversion fee applied by the bank of origin (in the UK).
A wire transfer in GBP would not be considered SEPA (yet).
"You must confirm"

From which UK bank you made a EUR SEPA wire transfer free of charge?
Tell people which bank if there is really one which makes them "free of charge" or don't spread misinformation.
I am saying that there is no charge for the SEPA wire in euro (obviously there is a charge for converting GBP to EURO with the bank in UK).
Also, Transferwise delivers fast SEPA wires to Paymium from the UK without triggering additionnal charges at the receiving end.