Well the sticker comes from the EAPC law, its on here:
https://www.gov.uk/government/publi...cles-eapcs-in-great-britain-information-sheet So I'd expect if someone was stopped, and asked to show that sticker if they didn't have it then the bike can be seized.
I've seen those stickers, you can buy them on sites next to the 'Vaccine Exempt' cards and 'I am Service Lion' collars
But in all seriousness there is a way to establish. I had a diagnostic done on my bike recently, its something most IT Techs should be able to do as its plugging a USB lead between my bike and a laptop then running some software. I'm looking at the report now and it lists some important points like Type = Pedelec and Max Speed For Support = 25km/h.
If bikes were seized, and examined, its incovenient for the owner but they'd get the bike back. BTP and TFL probably (I have no proof of this) just decided that they don't have the resources to do that so a blanket ban just seems like grabbing the lowest hanging fruit and calling it a win.