Excel is made of glutaraldehyde ( similar to formadehyde) which is very liight sensitive. This is why it is shipped in opaque bottles. From what I recall when I looked into this issue a year ago, Fisher Sci has breakdown info in the MSDS that suggests a significant decrease in concentration is detected in 24 hours.
So yes it will theoretically speed up the degredation reaction, but it may not speed it up any more so than the 4 wpg you are already using. There may be no practical decrease in effective concentration given that you are following directions to dose every 24-48 h .
Net benefit of the UV sterilizer may outweigh the degradation.
Simple test for you, plant some rotala indica about 3" high which many of use have noticed will grow only so -so with Excel (it goes full tilt in injected tanks). Then measure the stalks with a ruler, as well as a few braching leaves, let tank go 1 month with UV and excel, then measure and examine algae growth. Then second month turn off UV, cut off a 3" stem and replant, at end of second month , measure new and old stem for growth rate, note any algae growth or return. Consistenly add Excel at 1ml/10g every 48h thoughout experiment. Do your best not to make dosing, feeding , fish load or other changes in two months.
I think you will do everyone a favor, and soon be an expert advising the rest of us if you have the patience to do a 2 month exp.