The age of consent in the UK is 16. This means that it's against the law for someone to have sex with someone under the age of 16. It wouldn't be illegal for someone who's 16 to have a relationship with someone who is 30 - unless that person is their teacher or in a position of authority.

In the United Kingdom, the legal age of consent for any sexual activity is 16, regardless of gender. This means that a relationship between a 16-year-old and a 40-year-old is technically legal under the Sexual Offences Act 2003, provided the sexual activity is consensual and there is no "position of trust" involved (e.g., the older person is not the 16-year-old's teacher, social worker, or sports coach). However, while it may be legal, such a significant age gap is often scrutinized by social services and police as a "safeguarding" issue to ensure the younger person is not being groomed, coerced, or exploited. If the older person is in a position of authority, they could be prosecuted even if the 16-year-old consents. Additionally, many social and child protection organizations highlight the inherent power imbalance in such relationships, urging caution as the younger individual is still legally considered a child in many other contexts until they turn 18.