Wait. I know this place, and this is not accurate. Dogs are NOT allowed in the Lido, except in a short break between swim seasons when it's closed to the public.
The Lido summer swim season runs until September 12, and is then closed to the public. The dogs can then swim in the Lido at weekends from September 12-October 6. Owners can get into the pool with the dogs. But the general public are not allowed; you have to be a dog owner and buy a special ticket in advance.
After Dogtember finishes, the pool is drained, cleaned, and refilled.
The reason they do Dogtember is because it raises about 250k for the Lido, which is a charity.
Also, doggies really enjoy it! There are lots of photos of them looking so happy having a nice swim!