Well this is no help I think, but I recall now because I had a 6 character registration I was able to use a smaller plate, if yours is 7 that may stop you using the smaller plate that is offered. But if yours is 6 then look around as that apparently lets you use a 1950's size plate which is smaller.
Otherwise you'll have to shop around for something or try and blag a supplier into offering smaller than standard characters.
The standard UK plate is, aesthetically in my opinion too big for the car, so it might be worth trying the smaller characters - you'd be jolly unlucky to be pulled. Really can't remember who did mine but they were a mirror silver on black plastic.