(Sorry for the late reply)
Well, it doesn’t seams to be that simple.
The IAPP (International Association of Privacy Professionals) has a very long paper on the topic:
In short (and also based on some information I read somewhere else, but I do not have the source of it anymore), there is quite a big problem of timing.
A 3th country can request the EU to be concidered “equivalent” to the EEA-countries concerning privacy-protection, but the Commision has already stated this means that the UK can only … once it is a “3th country”, i.e. AFTER brexit.
This means that it is simply impossible that the UK will be concidered “GDPR equal” at brexit-day+1.
Also keep in mind that the EU is not in a hurry to provide that status to the UK. Appparently, almost all big companies that operate from the UK and process data of EU citizens have already moved their data to datacenters in the EEA. (wirex seams to have chosen not to do that).
Now, if you read the document, from the EU point of view,there are issues concerning access to data about EU citizens on datacenters in the UK to (e.g.) the police and the intelligence services in the UK. So -as the EU is now in the stronger position- it has every reason not to use that card to fast.
So it could be that this discussion can take quite some time.
Anycase, as I see it.
If it turns out that -at brexit-day +1- -say- visa has blocked your card, my guess is that it does not make sense to contact a helpdesk in the UK, as I do not see any way how visa Europe, any payment processer in the EU/EEA or their bank in Lavia will be legally able to share information about your transactions with what is then “an organisation in a 3th country”.