Parliament is sovereign and it can make any laws it likes. In other countries, Parliaments are not allowed to act so freely, e.g. some countries with written constitutions do not give rights to Parliament to override some constitutional articles without a referendum, whilst some articles cannot be repealed at all such as in the Russian constitution. The UK has nothing like this, whether the article in question is a human right or simply a secondary relatively unimportant regulation, the law can be repealed by the 652 MPs collectively. Hence, Parliament may do whatever it likes, this is one reason why it is the policy of parliamentary sovereignty is important to UK and not, governing bodies such as the monarch, the prime minister or any other ruler.
Also, no Parliament can bind future Parliaments, save in a few situations. Parliament is sovereign at any time that is present, so whatever the earlier Parliament said does not necessarily bind the current Parliament and what Parliament says now, does not bind the future Parliament. Even if current Parliament wanted to bind the future Parliament and presented it as expressly as possible, the law could easily be repealed.
Is parliament not sovereign?
The UK joined the European Union in 1973 by signing the European Communities Act 1972. The European Community law has to be given primacy over domestic UK law under requirements of the European Community Law.
The institutions within the European Community are allowed to make laws which the UK has to apply in its courts as long as it affects the UK regardless of whether Parliament wishes it or not. However, as previous Parliaments decisions do not bind future Parliaments the UK can withdraw from the European Community if they so wish it and the political parties have often use the European Community as a tool to win elections promising the electorate referendums. But this would fare drastically mostly for the economy, which is the back bone of every country.
The Bill of Rights in 1689 did not as much lay out rights of the people in the UK but restricted the rights of the monarch. The UK have negative rights which means than instead of having a ‘Bill of Rights' that tell us what we can do we have statutes and common law that proclaim what we cannot do. The Human Rights Act 1998 incorporated into British law is a first of its kind and is derived from the European Convention of Human Rights that had to be adopted by all the member states of the European Community. Thus it is harder for Parliament to exercise its sovereignty in such situations where the political and civil rights may be infringed by legislation being proposed.
However the Human Rights Act 1998 is not entrenched so it can be repealed through Parliamentary sovereignty although again with a caveat of consequences.
Devolution and establishment of the Northern Ireland Assembly, the Scottish Parliament and the Welsh Assembly does perhaps affect the sovereignty of the Parliament at Westminster. It would be difficult for Parliament to get rid of laws that these devolved powers have made within their legal jurisdictions short of dissolving the devolved powers altogether. But many see devolution as a good system of government because the participation level of the electorate within these devolved governments in their communities could be higher as they are much more local than Westminster.
Another general threat to Parliamentary sovereignty is the purposive approach that judges have started to adopt in the interpretation of statues. The courts had to implement this approach in interpreting European Community Law under the European Communities Act of 1972. Hence, judges focus not mainly on the strict view of law but interpret it keeping in mind what legislators would have intended.
Taking into account the loss of Parliamentary sovereignty through the membership of the European Community, the incorporation of the Humans Act into the British Legal system, devolution and the purposive approach, Parliamentary sovereignty is indeed declining but perhaps the idea that Parliament was ever completely Sovereign is questionable as the Members of Parliament have been accountable to the electorate in every general election. But because of majority that the government has in the House of Commons it becomes easier for them to pass a Bill once the party whip indicates to the rest of the MPs within the party which way the Party leadership would like them to vote such as in the Prevention of Terrorism Act 2005.
In conclusion, although lack of Parliamentary sovereignty can appear to be both progressive and illiberal, the former perhaps has a stronger case as we exist in a globalised world where combining forces for the progression of trade and economy is more essential than debating fading domestic sovereignty.