In 1988 Mrs Thatcher celebrated the ‘Glorious Revolution’ of 1688, in which the English establishment reached an accommodation with William of Orange whereby he would rule with Queen Mary in return for agreeing a power sharing agreement with Parliament. Mrs Thatcher was clearly wishing to celebrate the system we have now by association.

In the meantime however, the balance of power has evaporated. When George I appointed Robert Walpole as the first Prime Minister in 1721, it can hardly have been envisaged that the sovereign's power to overrule, or at least challenge the Prime Minister would disappear, but that of course is what happened. Couple that with a rigid party system and our first-past-the-post voting system, which almost always delivers single party government, and you have a Prime Minister who is a virtual dictator until he or she is toppled.

MPs on the government side do of course sometimes 'rebel', but very rarely does such a rebellion lead to a government defeat on an issue it considers vital. The rebels calculate enough will remain loyal to carry the day. Why do backbenchers not defeat the government more often on issues they feel strongly about? It's largely self-interest. They want jobs in government and to be reselected, and fear the indiscretions the whips might use against them. However there is also support for the 'tribe' they belong to. There is an element of self interest in that of course. They want their 'tribe' to go on governing. But there is also an emotional attachment.As W.S.Gilbert had it:

When in that House M.P.’s divide,
  If they’ve a brain and cerebellum, too,
They’ve got to leave that brain outside,
  And vote just as their leaders tell ’em to.
But then the prospect of a lot
  Of dull M.P.’s in close proximity,
All thinking for themselves, is what
  No man can face with equanimity.
  Then let’s rejoice with loud Fal la – Fal la la!
  That Nature always does contrive – Fal lal la!
    That every boy and every gal
      That’s born into the world alive
  Is either a little Liberal
    Or else a little Conservative!
      Fal lal la!

Iolanthe, Act II, Private Willis' song, v. 2