Personally, if I were to look to open a zoo anywhere, Brecon would be the place; an army town, with plenty of well off pensioners and families without many places to visit.
But Brecon is really just a small town, though maybe with those population extras you mention, and its pretty isolated too. I think to succeed you need to be within easy reach of at least one city/conurbation, or preferably more. That's where Marwell scores- its within easy reach of three- and where Port Lympne falls down- its not near any really, just a few coastal towns.