Can a 31 year old date a 16 year old UK?

The age of consent in the UK is 16. This means that its against the law for someone to have sex with someone under the age of 16. It wouldnt be illegal for someone whos 16 to have a relationship with someone who is 30 - unless that person is their teacher or in a position of authority.

Can someone 20 date a 16 year old?

No, its not against the law for a 20 year old to DATE a 16 year old. The definition of date means to go out together to a movie or to dinner or dance. It does not mean having sex regular or oral.