Personally I feel that there are plenty of opportunities of seeing wild dolphins in the UK and i wouldn't like to see them in UK zoos. I unsure as to any whales or dolphins being captivity as I feel no matter how big the tank zoos cannot truly look afteer a whale or dolphin properly (though I dont beleive the whales or dolphins are suffering in the tanks as I beleive the training often provides a welcome mental stimulus for these intelligent animals.)
This thread seems to be nothing more than personal wishes, speculation and guesswork.
I guess its the age old zoo debate how big is an ideal enclosure, like all animals dolphins and whales travel vast distances in search of food and a mate, in most cases not for the hell of it, so looking at it in that way a good size enclosure (and I'm talking acres here!!) would be fine depending on the species.
If an animal park was to keep these animals again I personally wouldn't have a problem with it if it was done in a ground breaking way, I saw a youtube clip earlier today of an aquarium in Japan that had a huge tank with several whale sharks and manta rays in it, it really was massive, it would need to be like that and then some.
Just over a decade ago it was ok to keep elephants in a small bland concrete yard, now its expected that they have sand, logs feeding devices, enrichment.
The same must be done for dolphins, huge tanks with rocks, sand, varying depths, feeding tubes so dead fish can be released into pools unexpectedly as enrichment, wave machines, hospital pens with raisable floors. Habitats for the animals to live in not empty concrete pools for them to exist in.
The cost would be phenomal but never mind eh! The technology is there for huge filtration plants, salt water instead of chlorine, underwater viewing tunnels, showing of the animals in their element not doing repetitve shows loosely based on natural behaviours.
If its going to be done do it the very best they can, not just an 70's / 80's throwback like windsor or flamingo land!