Depends where you go. Basingstoke aquadrome has a 25m pool mostly for lanes and swimming club; a 20m pool for lanes, school lessons, aquafit, swimming for all; and a separate section with slides and other fun stuff. For lanes at least, you can stay in longer than an hour these days - but you are expected to swim lanes, not mess about.
Newbury lido has a lane for length swimmers, the rest for children. Can't remember how long you can stay in, as I've not been this year so far. They also have an indoor pool, but I've never used that.
Petersfield lido is quite a small pool. Can't remember if it's got timed slots or not. I think it did. Tonbridge has indoor and outdoor pools. Again, I can't remember the restrictions.
I used to work as a lifeguard - we always ran with coloured armbands when it was busy in the holidays, and you wouldn't get more than an hour. (Unless you worked as a lifeguard and your kids got in for free - I did spend whole days up the pool, in and out of the water in my early '80s childhood, and Mum didn't have to worry about childcare, as we were there as she worked.) But pools are expensive to run. Councils are struggling to pay for all sorts, including essential services.
There's the sea, but you need to check if the water is clean.
It's not like Freibäder in Germany and the like, though. Some of them were quite chilly, mind you, at least early I the season, if it wasn't an all-year-round pool.