There is a company in Germany that I have used for high concentration (up to 300 mg/SL) in slaving an ozone generator to a municipal water process. I understand that that they have a full range of atmospheric monitors too ("smog" or similar application), depending on your need.
They use the Beer-Lambert method and UV absorption, so it will detect ozone, smoke, and "paint fumes" about the same.
There are electrochemcial sensors also, that are less expensive if you are looking to measure atmospheric ozone. The only downsides are there is a chemical cannister that will require periodic replacement, and the sensor will be responsive to (and depleted by) any oxidizer: chlorine, NOx, and or course ozone. I don't know of any European suppliers of these. In the USA:
... and I am sure they have some local representation in the UK.
If you are looking to measure ozone dissolved in water, I have another set of recommendations, but only USA manufacturers, sorry. ATI (above) have some good ones, and again they are locally repped.