According to the principle of parliamentary sovereignty, Parliament may make or unmake any law (in the form of Acts of Parliament), except those which would bind future parliaments.
As stated in the Supreme Court judgment for R (Miller) v Secretary of State:
“[…] Parliamentary sovereignty is a fundamental principle of the UK constitution […] It was famously summarised by Professor Dicey as meaning that Parliament has ‘the right to make or unmake any law whatsoever; and further, no person or body is recognised by the law as having a right to override or set aside the legislation of Parliament…”R (on the application of Miller and another) v Secretary of State for Exiting the European Union  UKSC 5
It is because of parliamentary sovereignty that the courts cannot strike down any Act of Parliament. However, the courts do have power to strike down secondary legislation when it has been implemented without proper legal authority (ultra vires).
Parliamentary sovereignty is partly a consequence of the UK’s lack of a written constitution. In countries which have written constitutions, the constitution is often considered to be the highest law in the land, and therefore cannot be easily overridden without special procedures being followed.