The above it a totally different thing altogether, If you import an old classic dub/yank/MGB in most cases you're not trying to pass it off as proper UK car are you, Same goes for the heritage shells, In the case of these Z's and their welded in bulk heads all you're doing is ringing the car, I doubt very much indeed if any of the owners are telling the DVLC exactly what they've done.
And if you do convert a US car and plate it up as the UK donor car then it isn't really a proper UK car is it it's just a fake/copy/ringer so I don't see how it has any value at all. I wouldn't buy one that's for sure, I'd rather have a properly restored genuine UK car personally.
Rob