@noisytoot There's indeed plenty of space compared to many other classes, but when run as this specific service the demands are much greater: people are more likely to be travelling with suitcases (very few commuters on XC trains). The luggage racks at the end of the coaches are too small for this, and although there's one large luggage area, nobody can get to it when the aisle is blocked, so chaos often ensues...
Originally, these were tilting trains, so the ceiling is lower than usual to allow the vehicle to still fit in the British Rail loading gauge. That also means that the overhead luggage shelf is far too small for suitcases, which makes the aforementioned storage problem even worse. But, to save costs, the tilting mechanism was deactivated years ago, so one doesn't even get a more comfortable or shorter ride than with other classes any longer!
The sets are much shorter than what you might expect for an intercity service, so they are still rather overcrowded and expensive IMHO.
There's a bad smell in the vestibules near each toilet, and the loos themselves all have suspicious brown patches on them every time I've travelled on these trains...
Finally, the seat coverings have, I believe, never been replaced since Virgin lost the CrossCountry franchise, so many of the seats are just totally worn through.
Everything is relative, but I'm amused that you picked the 700s as an example of a poor interior as those are some of my favourite UK trains 
(Admittedly, only for shorter journeys of ~20 minutes in my case)
First class is OK on the CrossCountry Voyagers, but really nothing amazing. If you bring your own thermos then I'd say Standard class on perhaps an Avanti set would be far preferable. I tend to spend half an hour extra waiting for a GWR service in preference to going on a Voyager as those are the two options for the Bristol-Plymouth stretch.