Thanks to all of you for your advice. I only had Tesco Metro as an option so I ended up getting Tesco Freespirit. It wasn't a fun evening, but no accidents! Feeling much better this evening though.
I was a little nervous when buying them, but I was very grateful for self-service checkouts. This afternoon I decided I didn't want to be in that position again, so bought some Abena M4s. I have been wanting to wear nappies for a long time, and this was a good motivation!