We're lucky in Oxford to have two swimmable rivers, though neither is currently swimmable due to flooding and high currents. Technically swimming isn't allowed in the lake I swim in at the moment but I think that's (a) to cover the council against any liability and (b) to protect the revenue of the outdoor pool in the same park, which is normally only open in summer and didn't open at all in 2020 - possibly they'd be keener on enforcing no swimming if it was open!

You might be able to find official open water swimming venues near you - I know there's one not far from Oxford though I've never been there. And rivers are generally ok if there's access to the banks.

(I do have to drive to swim, which is the one downside compared to the pool, which was on my way to work!)