You just beat me to it Dan as I was going to post a link to the manual. In section 2.4 Suspension it states "A missing bump stop is not a reason for rejection". Seems pretty conclusive that it isn't an MOT fail if one is missing.
Not allowed at MOT - unless I suppose you fitted two - on the same axle ! They are only permitted at low speed as emergency measure - which is why they are painted orange/yellow etc.
I don't think a missing bumpstop etc in itself will fail. but if the roadwheel can touch the body/chassis because its missing it will (may not be possible on a minor anyway)