It would certainly be a natural step on from the current conservation grazing already practiced on a lot of trust-held land in this country. Hopefully we will see the Wildwood trust do something with wisent, although I suspect they will work with re-created 'auroch'-type domestic cattle when they set up woodland reserves. But I agree wholeheartedly, the UK is particularly backwards when it comes to reintroducing species long lost from these islands considering the noise we tend to generate over the conservation of species in the former European colonies.
I wouldn't dismiss American Bison for their impurity, though. It's only our knowledge that it's there that really poses any problem. In fact, those endangered taxa that could successfuly recieve an infusion of new genes from the equivalent domestic stock should their populations get so low as to become genetically weak may have a distinct advantage over those with no such available genetic pool. Aren't our captive gaur all now genetically inferior animals due to inbreeding, much the same as babirusa, anoa and chevrotains are heading that way? True, right now numbers are not so low in the wild that we need consider such drastic measures. I just mean, if an ungulate species was in a real crisis in terms of numbers, and could produce viable offspring with a morphologically-similar domestic breed, what difference is it to us as long as we can bring the species back with enough of its genes to allow it to be phenotypically intact and able to behave and survive in it's original range following introduction? Certainly the herd of plains bison in North America fulfil the exact same ecological role as their (pure) ancestors pre-bottleneck. It could be argued from some studies that the Cambodian subspecies of Banteng is impure having acquired certain genes from the Kouprey, but if the mating(s) happen/happened by chance, it's hard to construct a solid set of criteria for evaluating the conservation value of a (sub)species based on its purity.
I'm still trying to find out whether Marwell did in fact have American Bison....I'm sure they were in the paddock currently occupied by Sitatunga, I would swear they weren't the european bison and that these were elsewhere, but then this could be a case of a failing memory, seeing as on another thread I revealed remembering a white-naped mangabey as a jet-black animal...