MPs can already claim first travel on expenses, providing it is for the purposes of travel between his constituency and the House of Commons (or at least they could before Legg). This to me is a perfectly reasonable expense. Not all MPs make use of it, some only travel standard class, and some refuse to claim travel expenses at all.
I say it is perfectly reasonable to pay them travelling because if we don't, then the MPs who live farthest from London are penalised, and lose more of their salary, making pay unequal, skewed in favour of London MPs who would get to keep all their money, while an MP from Scotland, the north of England, Wales, Northern Ireland etc, would have to pay out thousands of pounds, therefore ordinary people would not wish to stand as candidates in those places.
It is reasonable to pay first class because it is possible to reduce the cost by paying a season ticket or getting an early booking, and those on longer journeys can use the first class accommodation to perhaps use their laptop etc on the journey.
Democracy costs money. Accept it.
Some MPs haven't helped by saying things like "they don't want to mix with standard class passengers", but really it makes sense.