First way is devolution as it takes power away from the Westminster Parliament and spreads it to other parliaments and assemblies (regional authorities). Therefore regional parliaments and assemblies can make their own acts and pass laws and some acts of Westminster parliament are not affecting these regions. A good example of that is Scottish parliament with its own acts such as Scottish Educational Act and therefore almost all educational acts that Westminster passes doesn’t affect Scottish education at all.
Second way is that a lot of acts which are created in Westminster Parliament are passed because of government members majority and therefore coalition government members which also vote in parliament have the majority sits in parliament and therefore can affect acts and laws which can be passed. So parliament has lost sovereignty because government can affect parliament and they are not separate (like in USA where they have Congress and government and they are not bind together in any way) whereas in our system government is bind with parliament. A good example of coalition government affecting act passing in parliament are 37 acts, which has been passed because of majority of government members in parliament.
The third way is The European Union with rules that the UK must follow and abbey if it wants to stay in EU. An example of that are UK restrictions on prisoners voting rights. These restrictions disagree with EU Human Rights Act and therefore the UK has to allow prisoners to vote if Britain still wants to stay in the EU. This shows that Westminster Parliament is not sovereign as the EU can overrule British laws.