Certainly it is true that MPs nearly always do vote the party line. MPs respond to constituents' wishes in terms of making constituency representations, obtaining more resources for the constituency, taking up individual grievances of their constituents, but they don't take their votes in Parliament from constituents' wishes.
The previous government we had between 2011-2016 was in a time of great economic difficulty. There were increases in taxes and cuts in public spending. MPs knew perfectly well that their constituents didn't like those things, but the government felt it was necessary to do that to get the economy back on track, and the MPs went along with it. Many MPs would have said, “Well, if I do what my constituents want, I'll always be voting for lower taxes and higher spending”, and for that reason they do stick with the party line. Constituents don't really expect their MPs to follow their views on policy stances; they expect their MPs, really, to be pretty loyal to the party line in government.