Staying Legal and Respecting Others
You can usually swim in places where there is established use. So anywhere that is common land that has public right of way like footpaths, lake shores, riverbanks and bridges. A lot of these wild swimming spots will have long established use and accepted as traditional spot. But, always check because unfortunately legal access is only given to some English and Welsh rivers and lakes.
I don’t want to sound like your moaning nan but respect other people that use the waters too. You will from time to time come across people fishing, make sure you give them plenty of space particularly in the mornings and evenings when people go fishing like there is no tomorrow. Fishermen can be annoying, but it is also their river too and they pay to use certain rivers, lakes and reservoirs which ensure the waters are clean and pollution free. Which is great for the environment and wild swimming. If there are “no swimming” signs then I would recommend respecting that and moving on, there are plenty of spots where you can have a nice swim.