I think E marks are a 'Constuctions and Use' thing, rather than MOT 'inspectable'.

So if you were unlucky enough to be pulled for a VOSA check or a very over enthusiastic plod, they would pick up on non E marked lights, but the MOT would not.

1972 4/4 4 seater, 1981 MGB GT
1984 Harley Davidson Electra Glide, 1990 Kawasaki ZX10