A couple of thoughts.
I see that the proposed diecast was 1:64. Personally, that's way, WAY too small to be of interest to my middle-aged eyes. I wouldn't even have bothered with a 1:43 scale. That's still too small. Another personal preference but I also would have preferred the more unusual TR4 used in the pilot and first season.
Revell recently made a very nice 1/18 scale TR4 diecast. I don't know if it's still in production (seems to be in short supply on the net).
I'd explore the possibility of making a deal with Revell in the same way that others used to make make deals with Revell-Monogram to make a production run of monster kits and sci-fi kits which they would then sell under their own label.
Revell already has the tooling for a nice 1/18 1963 TR4 which they've sold in various versions and colors. I'd see if they would be interested in selling you a run of white TR4 street cars which you could put in a collectible MFM boxing. BIG BONUS: your kit and Uncle Martin figure (which I have built BTW...great kit) are in matching 1:18 scale. That's got all the makings of a perfect diorama.