The Tooting Bec Aquathlon evening series is open for registration. There's four events on Wednesday evenings through 25 May - 7 July. It's a 1000m swim in Tooting Bec Lido and 7.5k run round the common. I've found it good fun and easy to get to in previous years, and I've registered for a couple this summer - 8th June and 6th July.
Would be great to see a few other Windrush down there :-)