The manufacturers should have information about the light transmission with both the poly and the glass so you could compare. Glass can vary quite a bit in the amount and type of light that it lets in so even though it looks so clear, sometimes it is surprising how much light is being blocked by glass.
Even though you are in England, you should be getting lots of light in the summer time and you won't need to worry quite as much about light levels in the cooler winter with plants inside or dormant.
Your challenge will be spring/fall when greenhouse temps can be high in relation to light levels. I think you can help combat etiolation and such by just waiting for the first water and cutting it off in the fall for the winter.
All Cacti are succulents, but not all succulents are Cacti