What make/brand are these roof lights?

Before you do that, spray a small spot with WD40 or similar and see if that improves the transmission of light. If it does not, then repolishing the face won't help much as the UV light has damaged the full depth of the plastic.

I love a challenge

Sorry UK but do these look possible?

I think you'd change the curb around them as well a the lights themselves.

I had an old plastic window that was completely opaque - could not see through it - this made it like new again.

https://www.amazon.co.uk/Renovo-International-Plastic-Window-Polish/dp/B002G274WC

There are lots of other very good plastic glass cleaners out there - watch makers use them - but this one worked for me.

This one I've heard is good - but not tried it.

https://www.micksgarage.com/d/paint-polish-and-wax/products/537895/meguiars-plastx-clear-plastic-cleaner-and-polish

OP, in case you hadn't realised it it seems unlikely that the current roof lights were the original ones.

There seem to be two curbs one for the original roof light and another for the replacement thats sat on top.

So if you are replacing them you might as well look into getting rid of the second curb and fit the roof lights on the orignal curb. Unless of course that is some obvious reason I'm missing for them being done that way or they are original.