Originally Posted by
alanR
Having a visa doesn't guarantee entry. Therefore they need to take the information used to get the visa in the first place
That's true, though under recent changes to legislation and regulations, a visa is taken as conferring leave to enter the UK. Therefore, while possession of a visa doesn't guarantee admission, it would be refused only if the immigration officer had reason to believe that the person's circumstances had changed.
It is wise to have the documentation to hand, but it is highly unlikely that it will be asked for.