My local pool (Putney Leisure Centre) is a 33 1/3m pool although they often have the lanes set up sideways using the diving pool bit (it's an L shaped pool) and so the lanes are 24.5m.
I appreciate open air swimming is a different beastie.
Indeed. Note that Swim Serpentine recommends, prior to the event, to be able to do either:-
* the full distance in open water
* twice the full distance in a pool non-stop
They also strongly recommend doing some open water swimming before the event, it is quite a bit different from the pool, the lack of mini-breaks at the end of each length soon add up, and you need to set your pace slightly slower to account for this. There are plenty of psychological factors to consider too (sharing the lake with fish, feet touching plants and other things, very low visibility in the water, water taste, swim biff, etc).
Also you'll be swimming in a wetsuit (or using a tow-float if not) and you don't want the event itself to be the first time getting used to that. Wiggle often have perfectly good wetsuits on sale for ~£100, Decathlon stock some too although I've no idea how good they are. Renting them isn't much cheaper if you're going to use them for a while to get used to them. Saying that, I rented one the first time I went to my local open water swimming lake and got advice from the company about which size would be best, I was then able to buy one on-line with confidence.
There are plenty of open water swimming places in the UK although they tend to only open up in May once the water temperature gets high enough (usually 12 deg C although each place may be different). I use Shepperton Lakes for my open water prep, it's also a handy 22km cycle ride away, and I can even go for a run around the lake if I really hate myself.
Reminds me, need to order a tow-float as I've booked a family holiday to Lake Annecy again this year and hope to nip off to do a longer swim each day and there are a few motor craft that use the lake so I want to be as visible as possible.