Before the war, Morgan owners were trying to persuade officialdom that the numberplates should be bike size. They never did win the argument so car sized it is.
I've never seen a vintage trike with no front plate, although I have seen a bike sized one at the front, and a few with a rally plaque tied round and obscuring the front plate.
I have however seen a whole bunch of Grinnal Scorpions with no front plate.
I believe the fine for no front plate to be a small £30 but I do wonder if one was caught for this - with a personalised plate one might be cautioned that a repeat would lead to the withdrawal of the plate, as with naughty fonts.