I have not seen any evidence UV levels are linked to high humidity. If UV levels are lower after a cold front it may be there is more cloud.

Nicholas
Meir Heath, Stoke-On-Trent 250 metres above sea level.

If you have not seen the evidence, you haven't looked. There is plenty of evidence that low level humidity & high level UV are linked.

If you don't believe it, I can only suggest you look at Camborne UV over a few summers, & compare it with the synoptic chart. That doesn't prove the exact mechanism (Freddie, any help here) but it proves the strong correlation.