The Parliament Acts of 1911 and 1949

Up until the first decade of the 20th century, the House of Lords had the power to veto any legislation coming through the House of Commons. In 1909 the House of Lords refused to pass the "People's budget" of David Lloyd-George. Subsequently, in 1911 the Parliament Act (1911) was passed, plus restricting the House of Lords from vetoing any Act of Parliament – with the exception of one to extend the lifetime of Parliament – and also reduced the lifetime of Parliament from 7 years to 5 years (meaning that a general election should be held every five years). The Parliament Act 1911 gave the House of Lords an option to delay any Bill for up to 2 years.

The 1949 Parliament Act further reduced this period of 2 years down to 1 year. Both the 1911 and 1949 Parliament Acts work together. Since 1949, only 4 Bills have been passed using these Acts:

War Crimes Act 1991
European Parliament Elections Act 1999
Sexual Offences (Amendment) Act 2000
Hunting Act 2004

As a matter of convention, the House of Lords does not veto any Bill related to a matter mentioned in an election manifesto.

Another convention is that Parliament cannot bind any previous Parliaments. What this means in practice is that any new government that is formed may make changes or repeal any previous legislation from preceding governments thus ensuring that the legislative power of the government is not restricted in any way. You can read more about conventions of the UK Parliament here.

Bear in mind that conventions are also a source of law under the UK constitution and cannot be easily changed.