GG2 is in the right of it.
Under our constitution Parliament is sovereign. This is what has been established since the Civil War in the 17th century. This was fought to remove Absolute power from the Monarch (In an Absolute Monarchy the monarch alone was able to direct how the country was run and had sole power to make the law). This was regarded as tyranny by Parliament.
Parliament consists of two elements, the Executive (the government) and the Legislature. The Executive rules on behalf of the Crown but sovereignty rests with the Legislature. If the Legislature decides to overrule the Executive because it feels it is abusing its power and becoming tyrannical they have a constitutional right to do so because they, not the Executive, are sovereign.
The Executive only retains power as long as they have the confidence of Parliament and can pass legislation with a majority vote in Parliament.
We have a party system because it developed as the most convenient way for the Executive to be able to pass legislation but no party member is constitutionally obliged in any way to support its party when in government if they feel that their proposed executive actions are wrong. Clearly, if the government (Executive) has a large majority they are able to pass legislation even if a few of their MPs dissent and fail to vote in support of them. Also, when the Legislature (the House of Commons) scrutinises proposed legislation any MP, from any party, is able to propose amendments to the proposed legislation and it can be changed if the Commons votes to accept the amendments.
A prime duty of a member of parliament is to consider the good of the country when scrutinising proposed legislation. The 'will of the people' really doesn't come into it. If MPs consider that proposed legislation will be damaging to the country they are perfectly entitled to make that consideration their prime motive for opposing the Executive.
Considering 'the will of the people' is a political consideration, not a constitutional necessity.
At the moment we have an Executive which is apparently determined to disregard the wishes of the Legislature by pursuing a 'no deal' Brexit despite the fact that it has no support for it in Parliament (and, only minority support in the electorate). It is proposing to do so by abusing its powers. MPs have every right to attempt to prevent it from succeeding and to defend the sovereignty of the Legislature.
This, like it or not, is how our Parliamentary democracy works and no amount of frothing at the mouth and shouting about 'democracy' can alter it.
As for a 'Party of Unity', the idea is relatively new and politics will be involved in sorting it. It would be unrealistic to expect it to emerge fully formed and with total agreement among its components right from the word go. If it is a goer differences will be sorted over a period of time. If they cannot be sorted then it won't succeed. But it is not unconstitutional or illegal in any way.