H. kuda - common, yellow, or Pacific giant seahorse. Found at Brisol, Chester, The Deep. The Sea Life group claim to have successfully bred and reared nine species of 'seahorse' (I wondered if they threw in pipefish to come to that figure),
H. kuda are not one of the easier species to rear successfully, and I would suspect they are declining in the UK. They can be a very colourful species and were common in the aquarium trade until the 1980s.
To clarify about the other species on display in the UK:
H. guttulatus - Long-snouted, or 'Spiny' seahorse. Previously referred to as
H. ramulosus - this classification no longer exists. This is the most common of the two species found in UK waters, and the one that the Sea Life group make much of having bred well in captivity:
Back from the brink ? the Spiny Seahorse
I'm in no way endorsing them, the link is just for reference. ZSL London zoo also have a couple according to ISIS.
H. abdominalis - 'Australian big-belly' or 'Pot-bellied' seahorse. This is the other species imported during the late 90s by the sea life group. As you will see from ISIS, the less corporate zoological collections don't keep these. Sea life aquaria retain sizeable groups of this very large species. They are very easy to rear as, being bigger fry, they can eat Mysid shrimp at a younger age (far more nutritious than even enriched artemia hatchlings).
H. hippocampus - Short-snouted, or 'Atlantic' seahorse. This is the other British species, but needs the warmer currents from the gulf stream and stays south. Found at some Sea Life Centres, ZSL London Zoo, and Living Coasts. I think the Seahorse Trust also maintain some at The National Aquarium. Doesn't breed so well in captivity, many in aquaria are those caught accidently by fishermen.
H. reidi - Slender, coloured or long-snouted seahorse (WMSP, Some sea life centres). This species was one of those prioritised by 'Project Seahorse' in the late nineties, however in captivity they are a very hard species to rear, as a result I strongly suspect this species is declining in the UK, would love to be proved otherwise:
BBC NEWS | UK | Scotland | Glasgow, Lanarkshire and West | Breeding woes for lonely seahorse
H. erectus - Northern, or 'lined' seahorse. Found at London Aquarium.
H. fuscus - The 'Sea Pony'. The species most commonly found in the aquarium trade. Fairly easy to rear in captivity (I bred them successfully years ago). Found at London Aquarium, The Deep, and some sea life centres, but significant numbers I believe to be in private hands.