Dear terrine,
Unfortunately my main area of expertise is not dairy, unlike Caz.
The retailer has suggested that UV is insufficent to destroy Pseudomonas
.
Well, I have no direct UV experience but I think it’s fairly well documented that UV is capable of inactivating typical Pseudomonas species in drinking water, filtration applications.
However, from memory, UV may have certain design limitations, for example regarding efficiency versus volume flow-rates / pipe line dimensions. Yr installation agent should presumably be able to validate any such practical aspects.
The water is only used for cleaning not an ingredient
The problem here is that any contamination of the water will potentially interact with the dairy processing environment ?. And Pseudomonas is clearly a well-known, potential, major headache in the milk business from such opportunities.
It may be simply that yr retailer has a specific list of minimum requirements established from local industry “standards” / problem experience / internal QA which include chemical control as a “must have”. Caz’s post suggests this may well be the case (ie prevention better than cure, especially if someone else’s $). You presumably see other similar operations to yr own, how do they compare ?
IMEX (non-dairy, no particular Pseudomonas problem) I was long time ago offered ClO2 since customer loved the stated, non-tainting capability, but it was significantly expensive plus other options were of adequate microbiological efficiency / product flavour. Horses for Courses. 
Rgds / Charles.C