Cardholder validation only works for recurring payments
When trying to make a donation, cardholder verification will not happen unless a recurring payment is being set up.
Specifically, for non-recurring payments, it will show that it is pre-approving, and then will fail if the bank requires validation.
For recurring payments, the validation window (specifically the mastercard id check in my case) will (eventually) show and the transaction can be completed.
This means that for most users in UK, non-recurring payments will always fail.