Sorry, I thought it was a universal term. A lido is an outdoor pool with sheltered sunbathing spaces. Named after the Venice Lido (which is actualy a beach).
There were a lot built in London during the 30s - an alternative for people going to the beach and taking their money out of town. They were out of fashion in the 80s and people had to fight really hard to keep them open. Some have survived, most famously the
Serpentine and
Hampstead Heath, although I used to go to
Tooting Bec (the largest fresh water pool in England).
We don't really have the weather for outdoor pools to stay solvent - they're unusable for at least 2/3 of the year.