But, they tried to fob me off, with some supporttard telling me to clear cookies and cache (classic fobbing-off technique), and closing the ticket. So I do now have to treat the funds as stolen.
Support provides solutions such as a technical error, just hope this isn't their attempt to buy time which in the end considers it to be a problem with the source of funds or an account security risk.
What if you exchange some of your GBP for bitcoin and then try to withdraw it?