How is parliament still sovereign?

Following successful referendums, Parliament devolved substantial power to the newly created Scottish Parliament, Welsh Assembly and Northern Ireland Assembly.
The existence of the ‘Sewel Convention’, as it has become known, was recognised by the Scotland Act (2016), and, despite the fact that Parliament could legally pass laws in devolved matters without first seeking approval, it has always waited for a ‘legislative consent motion’ to first be passed.
This highlights that, in practice, devolution acts as a considerable constraint upon the laws that Parliament can now pass. While the devolution Acts could be repealed, and power restored to Parliament, this is arguably politically unthinkable without first holding a referendum.

Section 3 of the Human Rights Act (1998) requires judges to interpret all UK laws, as far as is possible, so that they are compatible with the European Convention on Human Rights.
If this proves impossible, Section 4 of the HRA requires judges to issue a ‘declaration of incompatibility’.
While this declaration does not strike down the law, it does place Parliament under considerable political pressure to make necessary amendments. To date, Parliament has usually responded quickly to ensure laws are compatible. Section 19 of the HRA also requires government ministers to issue a statement before the Second Reading of any bill they introduce, explaining whether it is compatible or incompatible with human rights. Although these statements are not legally binding, they act as a reminder that, as long as the HRA is in place, ministers are significantly limited in the bills they can propose.

The fusion is efficient because it gives the executive considerable control over the legislative branch.
The Prime Minister, along with the other government ministers can all vote in favour of government bills.
As the convention of collective responsibility requires government ministers to support government bills, this means that the government can rely upon as many as 118 votes even before an effort is made to convince other MPs to lend their support.
The fusion also allows the Prime Minister to reward loyalty from other MPs, by directly promoting them into the ranks of government. This greatly helps the whips to keep backbench MPs supportive and means that a government with a sizable majority in the House of Commons can be confident that its bills will not be defeated.
As the House of Lords cannot veto bills, and the monarch is bound by convention to always sign bills, and the Supreme Court is unable to strike down Acts of Parliament, a Prime Minister with a large majority in the Commons can enjoy considerable power.