I don't think compulsory reservations for trains in this Country would work very well at all. In Spain their long distance trains are all compulsory seat reservations. Once the train is fully booked, the website states "Train Full" and no further bookings are taken. They guarantee a seat this way. But the lifestyle and working culture out there is very, very different to that of Britain.
I think
FGW▸ would do good to do what Virgin have done with their Voyagers. Have a section that can be alternated between First and Standard Class as required by demand. Doesn't necessarily need to be 2+1 Seating but be only Tables of 4 like
Mk2▸ Carriages.
Or offer a scheme similar to GreaterAnglia with a First Class Upgrade on Off Peak Trains for ^10.00 Return. In FGW's case maybe knock that up to ^15.00 or ^20.00 to factor in the longer distance and complimentary refreshments.
Introduction of First Off Peak and First Super Off Peak may very well sway people to upgrade too. I personally won't pay the price of the
FSR▸ (First Off Peak Return) between Frome and London Paddington based on the fact price paid doesn't reflect the service gained on Off Peak Trains. the 19:Something from Westbury - London Paddington Stopping Service is a prime example of that! A train which is fairly empty in Standard and First alike
If people are going to use the train then they've taken the Standard Class revenue anyway, but if the train was very busy or a Family of 4 wanted to sit together then they may very well be tempted to upgrade onboard to be able to sit together or have peace and quiet.
What I am wondering is what is going to happen to the Quiet Carriage in First Class? If some sets are only going to have 1.5 Carriages or First Class. Putting the Quiet Coach in the Buffet is a bit silly and will no doubt upset people when the Pullman is laid up. I personally would be rather peeved if I boarded a train to find .5 fully taken and the remain coach was a Quiet Carriage.
A few months back I got into a hostile situation where a chap was telling me to stop using my mobile phone in the Quiet Carriage. I ignored him stating it wasn't the quiet carriage. We had 2 coach Gs in the consist. I'll leave you to make your mind up to which coach I was in...