The enclosures at Yorkshire Wildlife Park are indeed very impressive and it was very close for me to consider in comparison to Folly Farm but the lion housing at Folly is greater with multiple large dens and large yards for the lions in comparison to the den or so for each lion group at YWP. That and the second enclosure for the same group made me sway towards Folly but I do agree with you that the YWP enclosure is certainly very impressive and at a level most zoos should aim for to keep lions nowadays.
By the sounds of it I should have included Folly Farm as one of the stand alone options in the poll. Having never visited before I didn't realise how impressive their Lion exhibit was.