Update: Got the email from Natwest asking me to pay back my overdraft now that the switch has “finished”. Got until the 4th to pay, and I can use my card to do so. However, the website says it could take 5 working days to process, so I think that means I have until…today? 
Edit: I paid it using the shadiest looking payment portal I’ve ever seen. Good jorb Natwest. Now for the thrilling wait for it to show up in the account and them actually closing it…