So it turns out this was a legit message and I did owe this money. It was apparently a final amount minus a few days from my usual monthly bill and because I was mis-advised when cancelling to cancel the direct debit immediately with my bank. They forgot to tell me to wait 8 days for the final bill to be issued and debited which is why I got the message.
The customer services person I spoke to said the wording of "uncertain times" was probably meant to refer to the cost of living, and the general recognition that more people than usual in the UK are struggling to pay their bills.
Also apparently EE do not send bills out via email, which seems a strange decision in 2024. I have my confirmation email of the sim plan, so my email's in their system.
My home address was still an older one from last year, despite me remembering updating this - so I may have been sent a letter to an old address about all this but not an email.
Had I not had this sim in a backup handset that I just happened to be using last night for a moment I would have had no way of knowing that I owed money. To not send this message out to my linked email address and only be using paper and texts that I was right to be suspicious about seems a bit ridiculous!
This sim was for months in a 4G router. It's only in the last month or so that I put it in a phone while my main handset (different network) was broken. If it had stayed in the router or I'd chucked the cancelled sim away I would not have got the message at all.
If the message had only gone to a sim that was pending as cancelled then that would be bad, and would have probably racked up charges from me not paying the outstanding amount. If EE are going to follow other providers in being able to go paperless then they've got to use people's email addresses for stuff like this, unless it's a ploy to get charges to mount.
Anyway - rant over
I've paid the money
I wish I'd been explained these charges when I was cancelling to be honest
