Optional???
From the DVLA website:
And as the relevant British Standard for our cars is BS AU 145d, which states that for a plate to conform to this standard, it must be:
I'd say you don't have much to argue about really.
As for the Police Officer not having anything better to do, well...what would you expect - he's being paid to do his job, and it looks like he's....well.....doing it!
(Shocker!)
Sorry, Imi-7 but I've had illegal plates on vehicles too, but when I've been pulled up for them, I've just had to change them and on one occasion got a £30 fine which I paid straight away. Same as speeding - if I get caught going over the speed limit, how can I moan if a Police Officer does his job and reports me for it?
Don't take this sort of thing from the Police so personally. There's been a crackdown on illegal numberplates in many parts of the UK recently, so I suppose this sort of thing is to be expected.