OMG - why not?
When I was on FGW, in standard, I managed to sit quite comfortably without feeling cramped - and I also managed to put my laptop on the seat-back tray and open the screen up properly to use it.
Other trains may have more legroom, but if my legs aren't touching the seat in front (I'm 5ft 10in) and there's room to use a laptop - I am pretty sure that more room would be an unnecessary luxury, at the expense of a row or two of seats.
First class is obviously nicer, but I'm quite content in standard and thinking about what luxury I can enjoy later on with the money saved (bar exceptionally good advance offers).