Sorry to be bah humbug…I suspect as long as your number plate can be read by an ANPR camera you'll not face any issues.
All numberplates since 2001 have to comply with a British Standard number (currently BS AU 145d). I doubt the stick on plates from pro plates comply as they do not appear to state the manufacturer, this BS (!) number, comply with destruction tests or reflection requirements.
I guess you could take your chance, but the risk is £1000 fine and possibly an mot failure.
What fun living in a computer says no bureaucracy, as they look so much nicer!