If they stop you on entry to the UK they can compel you to unlock any devices you are carrying. They are entitled to whatever data they find on the devices but

If they stop you on entry to the UK they can compel you to unlock any devices you are carrying. They are entitled to whatever data they find on the devices but they are not allowed to use the credentials on the devices to access remote services. However, the secret services have a long track record of ignoring the rules so I wouldn't trust them not to.