Why does FGW not offer a Cheap Day Return to Reading?
Because cheap day returns are only for shorter journeys but you can still buy two for your journey to make it cheaper.
Not technically true- Cheap Day Returns are availible from Bristol T M to London Waterloo which I'm sure works out as *much* longer than Bristol PW - Reading.
Yes, it is very annoying, and those tickets should certainly be made availible. BPW-Reading isn't exactly a huge distance either. I remember reading somewere that you used to be able to get CDRs from Bristol to Paddington and Reading, but sadly times have changed and people are forced into finding loopholes in the system.