I love swimming and I love being outside. London is great if your into both these things. I swim at London Fields Lido which is heated all year round. Its best in the winter when the steam rises from the water and if your lucky its snowing a little on your head. If your brave you can also go for the unheated option of the Serpentine or Hampstead Ponds.
Half a mile from Markfield Park down the River Lee is Springfield Park. The park rises steeply up from the river and has an amazing array of tree species making it beautiful in all seasons. At the brim of the hill there is a line of park benches each one surrounded by box hedge offering a private view across the Lee Valley, Walthamstow and beyond. The park warden was nice enough to give me a chestnut log a couple of years ago from which I carved a bench and two stools.
Free natural ponds, open all year round (check online for varying opening hours according to seasons). Get an absolute get away feeling right in the heart of London, a treat if you enjoy swimming with ducks. Amazing park to enjoy a few games of cards and a walk around afterward too...