Generally, it is not illegal for these two to date. Of course, any sexual activity is considered statutory rape, but if there is no sexual aspect to their relationship, then they are probably fine. However, this is a fine line to walk for the 21 year old.

Is a 16 year old dating a 21 Legal UK?

16 is the age of sexual consent in the UK. So anyone over that age can legally date, or indeed shag, whomsover they choose (unless the older person is in authority over them - such as a teacher, or a family member) Nothing prevents a non-sexual friendship younger than that. It is illegal to pay anyone under 18 for sex.