Having said all of that, and judging from the miniscule amount of driving that can be done in the U.K. in comparison to the U.S.A., it seems possible that I'd be able to tour 5 small British zoos every day if I was to fly over to England. A major establishment like Chester would be all day; I've several zoo fans that have toured London Zoo in 5 hours and then maybe one other attraction; and then surely there are days when I could see 15 zoos in 3 days provided all of the facilities were tiny? Is that accurate? I'm asking in all honesty because if I travel to England five years from now I'd spend a year preparing an extensive itinerary. I'd like to see many major zoos with one or two per day but the tiny size of the country makes me think that 5 or even 6 one-hour collections is possible if some of them have later hours into the evening.
To do 5 collections in a day you would *really* have to rush most if not all

the majority of collections you'd be prioritising would require a decent amount of time devoted to them, and UK collections tend to open later and close earlier than continental European and US collections do.
To give you a general picture, here are some of the UK collections you are likeliest to prioritise, and a suggested "minimum visit length" - naturally some can profitably take a lot longer in order to see everything, but I know how you like to squeeze things in!

for the purposes of this exercise, take "6+ hours" to indicate a full day, as depending on the time of year a zoo may only be open for this long, or less.
Blackpool Zoo - 3 hours
Bristol Zoo - 3 hours
Chester Zoo - 6+ hours
Colchester Zoo - 5 hours
Cotswold Falconry Centre - 1 hour
Cotswold Wildlife Park and Gardens - 3 hours
Crocodiles of the World - 90 minutes
Edinburgh Zoo - 2 hours
Exmoor Zoo - 2 hours
Hamerton Zoo Park - 3 hours
Highland Wildlife Park - 5 hours
Howletts Wild Animal Park - 4 hours
Living Coasts - 90 minutes
Marwell Wildlife - 4 hours
Newquay Zoo - 2 hours
Paignton Zoo - 3 hours
Port Lympne Wild Animal Park - 5 hours
Scottish Owl Centre - 90 minutes
Shaldon Wildlife Trust - 1 hour
Slimbridge WWT - 2 hours
South Lakes Wild Animal Park - 2 hours
The Wild Place - 90 minutes
Twycross Zoo - 3 hours
Woburn Safari Park - 3 hours
Yorkshire Wildlife Park - 3 hours
ZSL London Zoo - 6+ hours
ZSL Whipsnade Zoo - 6+ hours