So, I bought my yellow burner XR and visited an EE store to get a new eSIM. They didn't have any eSIM packs available at either of the stores, so a physical replacement SIM was the best they could do...
Idiot handling the request only went and killed my Apple Watch cellular eSIM instead! Sadly, I didn't find this out until this evening when I couldn't activate the new SIM and noticed my Apple Watch was no longer connecting.
Can't send my XS Max off tomorrow until I have some way of getting connected. Thankfully, I have until Tuesday 24th to get my phone to Music Magpie, so there's still time.