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.
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.