The answer is No and No. A 15 year old cannot legally have sex with ANYONE. The 16 year old could. This means that the 16 year old can be charged with statutory rape and sentenced as a sex offender requiring among other things

Is it illegal for a 15 and 16 year old to date UK?

Whats the law about under–age sex in the UK? According to the Sexual Offences Act 2003, its a criminal offence for any kind of sexual act to take place between two people where one or both are under 16. It makes no difference what your gender or sexuality is, the law will be the same.

Can a 16 year old be with a 15 year old?

It doesnt become illegal for you to date your boyfriend when he turns 16. The law doesnt talk about dating at all, because dating can mean different things to different people. People can become ready for sex at different ages, but the law says that if someone is under 16 then its illegal to have sex with them.