Some more info:
I called Barclays (number on back of card), who put me through to Barclaycard, who then put me through to the anti-fraud team. It appears the latter team deals with 3DS verification and so on.
They fed me the 'additional 6 days' before "all online transactions" will work properly and that it is a matter of sitting tight and bearing with them.
Barclays are (supposedly) a well-established bank in the UK. No wonder Monzo, Starling and Chase are so popular these days.