David, you would be well advised to visit Howletts if you want to see dhole in the UK.
They have a decent sized pack, and Bekesbourne (near Canterbury - worth visiting if you have even a cursory interest in English history) is a lot easier to get to from London than Galloway. Twycross only has a pair; having not visited Port Lympne since the place was pulled apart this spring I have no idea about this species' status there.
Sadly easier said than done being based up in County Durham, everywhere which holds Dhole is highly unfeasible to visit for me - going to Howletts would cost me 120 quid on the train, and I'd only get 2 hours there before having to set off home again Twycross - being not *too* far from Manchester where my gf attends university - although badly served by public transport, is the only one that I have any chance of visiting.
