Banks are allowed to charge for Euro transfers within the SEPA area as long as the charge is the same for domestic and SEPA transactions. As the UK doesn't use the Euro, our banks can have a zero fee for domestic transactions and another for Euro transactions. However, you should find it cheaper than the alternative SWIFT transfer (my banks fee for SEPA transfer is half the cost of SWIFT)
If the Halifax sends in it euros, they will do the conversion before sending the money. If they send it in GBP, the receiving party will convert it. The choice depends on which gives the best exchange rate. If it was me, I'd send in euros unless the receiving party is happy to get GBP and won't charge for converting them.
Thanks for that, clears it up somewhat