Thank you so much tatt. You are a real mine of information!

I've visited this swimming pool in Axminster that is UV treated so is low chlorineFlamingo Pool. It has sessions with an inflatable so extra fun for young children. You can stay in nearby Lyme Regis (beautiful), Charmouth (for fossil centre) or Seaton (cheaper, has only a pebble beach, but has the Tramway and a land train to Beer for Pecorama if your son likes trains).

In London some clubs have ozone treated pools - not sure if you'd get in to The third space or Soho: City Point Club.

Spas pennyhill park and Chewton Glen and Stoke Park Club

Not sure about the Ozone Leisure and Entertainment Park, Oxford - you could contact them to ask. There will be an ozone pool at University of Surrey for when he's older .

My younger DS (nearly 3) has had a horrible time with his breathing (including a stay in hospital with pneumonia on his last holiday).
DH has asthma, and doctors are not sure if DS does too.
I have decided to keep him away from chlorinated pools as they can apparently increase the chances of a child developing asthma.
Sadly, there are no non-chlorine pools near where we live so he is missing out.
I wondered if anyone has been on a holiday somewhere with a non-chlorine pool.
(I have also posted this in the General Travel Advice topic - sorry if you've read it twice).