Longleat is VERY hot on keeping windows closed in the danger zones (monkeys, lions, wolves etc) - if you see a zebra striped Land Rover they will be on their megafone damned quick. Best advice for these areas (I live virtually next door & visit regularly) is clean windows thoroughly, keep them closed & butt your lens right up against the glass.
Good tip is to do the interesting animals early or late in the day as they are much less active in between.
The butterfly house is cheap (£2 each), intersting but smaller than you might imagine. It's also like a sauna.
Meerkats are also a good laugh - they have a new young family there, but they may be kept indoors if the weather is really bad.
Have a great time!