In centralish London your best bet is one of the lidos, which are basically big outdoor swimming pools. E.g. Tooting Bec Lido is 100 yards long. You can also swim in the Serpentine lake in Hyde Park, there's a designated area which is also 100 yards I think. Most of the lidos have clubs associated with them who swim all year round, a lot of them will also be closed to the general public during the colder months (which includes April), so you may well need to join a club to get access but they're pretty cheap.

Further out of London there's more options for lake and river swimming, but again in April you may well find a lot of these are still closed.

Lidos are a good bet (Victorian-era outdoor pools). They vary in size (anywhere from 30-odd to 92m long), style (some are laned, some left “open”) & some are heated (e.g. London Fields, Hampton) whereas others are left to heat naturally (Tooting Bec, Brockwell, Parliament Hill). I wouldn’t recommend wetsuit swims in the heated ones!

Serpentine is great for sightseeing, less good for avoiding duck sh!t. Jokes aside, not the best for training.

Come later in the year (usually mid-May), the lakes will open. We usually swim at Shepperton (SW London), about 45-50 mins out by car/train/bike. They lay out a 400m or 750m loop. It’s increasingly popular but does double up as a great base for a long run afterwards, along the Thames Path.

There are a couple of other options for o/w (a reservoir in NE London) but I’ve not used them so couldn’t tell you much more I’m afraid.

Cost-wise, all the options above are between £5-10 per session, sometimes depending on what time you’re there.

Thanks for advice, was thinking more lakes and these sort of basin/dock things along the Thames more than Lido's?

Here you go, if you are nuts enough to swim in the Thames. All OWS in UK will be cold in April though!!
http://www.londonroyaldocksows.co.uk/


There is West Reservoir in Stoke Newington. Close to Finsbury Park and easily accessible by tube. Re-opens some time in April.
https://www.better.org.uk/...-west-reservoir-2017


Otherwise there are lakes a bit further out of town like Shepperton (closed until May) or Thorpe (re-opens 1st April).
http://www.sheppertonopenwaterswim.co.uk/
http://thorpeopenwaterswimming.co.uk/

Then even further out is Mytchett in Camberley, near Farnham, which is excellent but a bit of a trip. Home to Farnham tri club.
https://www.mysportingtimes.com/event/tri20-swim-centre


Good luck, Rich.

If you count it as 'in London' Shepperton is a fantastic location. The lake is very clean as it is never used by powered craft (unlike Thorpe for example.)