Just updating the situation, I received this emai on april 29th:
"Apr 29, 11:28 UTC

Hello!

Sorry for the late reply and inconvenience.
We'll make the refund on your card.
The problem will be fixed.
Kindly ask you to wait. You'll receive the refund withing 2-3 days.

Best regards
Wirex support team"

Yesterday I received this email:
"May 1, 13:58 UTC

Hello!

The refund has been done on your card. kindly ask you to check."

I checked my account and they deposited the GBP as I would have received if the operation went ok back then. So, wirex did solve it, but it is an inconvenience because it took a few days. Luckly, it wasn't urgent.

Since they are changing their exchange fee policies, I suggest make it clear how much is the fee for a given range of BTC being traded