Tentatively yes it is illegal.
You can't buy 'approved' reflective stick ons any more. The sharp will note the recent +4 Supersports model was supplied with one, but strictly - it will fail the first MOT on that basis.
Given the mark 1 eyeball will have no problem and I rather doubt ANPR will either, you'd probably get away with it.
My 1972 4/4 wears a paint and stick on number on the wing - and has since 1989 when the rules as they are now didn't exist.
1930 Super Sport Aero 'The Elk'