I think you are oversimplifying things.
1) N/a Supra is a great traditional sports car with good performance rather than a supercar-level GT like TT is. Performance of the n/a is similar than Porsche 968. And it wasn't cheap. Price difference in U.S market was 4000$ (34t$ vs 38t$) between base and turbo, and in japan it was quite similar difference. So they both were expensive cars all around the world.
2) Lesser models doesn't affect price, Porsche 911 for example.
3) Supra name heritage. It's a good name and Supra line has always been loved by people and the press, but earlier generations were a real mass products. There is a lot of different models too, and MKIV n/a is the second best in performance (after MKIV TT). So it does not affect overall value of the Supra's.
4) In britain there is a lot of Supra's that are in a very bad condition or chavved out*, and this shows in overall value of the car population. My Supra is from england and it was in totally shit condition. Mechanically sound but some chav did rape interior completely and forget the maintenance of the car. Now, after a lot of work and some money, it is far better shape and it is a cracking car. Even in this forum, some chap did think that 120£ is too much for ABS sensor. If you don't chance that kind of parts, it WILL lower yours car value. For example in germany RHD Supra's are much more expensive than in britain, and those cars are usually in quite good condition.
So i think that reason number 4 is the most important thing why price of the Supra is't that high in britain. In continental europe (LHD markets) even RHD Supras are quite expensive to buy.
*There is a lot of lovely and beautiful Supras in GB, like in this forum. I just did write things in general level.