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.
Late response but here you go. A couple of photos. The plates are 11 x 8" with silver on black acrylic. I simply drilled holes in the corners to match the holes in the rear panel. Then ran small screws through in each corner. The screws are in the silver border so by using silver screw heads they aren't noticeable till close up.
Then I got some acrylic small bore tube and cut it to length with a chamfer to fit nicely against the countours of the body. It worked well because the white of the acrylic blended in nicely. You might want to paint them body colour if your car is not white.
The steel screws then go through the plate, through the tubes which act as spacers and are retained by nut & washer inside boot.
It's not a concours solution but looks q good until you get within 18".
Sorry for got I'd post this Mine was mounted similar to Harry's but with out the spaces, just long ish black plastic headed nuts though number plate and into boot holes which appear to be from where I guess the plinth was, and bolted inside the boot