The age at which consenting adults are legally allowed to have sex is known as the "age of consent". Since December 2000 the age of consent for gays, lesbians and heterosexuals has been 16 in England, Wales and Scotland. However, in Northern Ireland the Age of Consent is 17.
Also note that the Sex Offenders Act 1997 makes sexual activity with a child, committed in an overseas country by an UK citizen or resident, a criminal offence punishable in the UK. It is also an offence to incite or conspire to travel overseas to engage in sexual activity with children.

PRACTICAL ADVICE
If you are over 16 it is illegal to have sex with anyone under 16 (or 17 in Northern Ireland). Remember that it is not a defence that you didn’t know he was 15 or he lied to you about his age.

If somebody looks young you need to be on your guard. Underage sex is "strict liability" which simply means if your sexual partner is under age then you have a committed a serious crime. Certainly, if you are in a relationship with a teenager I would strongly recommend that you check your boyfriend’s age. If you are embarrassed about this look at his passport whilst he is out or check his wallet for ID.

If you are traveling overseas the same rules apply. Just because you are not in the UK doesn’t mean you are beyond the reach of UK law and you could be liable to prosecution in the UK for child sex offences committed overseas.

If you are under 16 and you have a relationship with someone of or near your own age whilst technically you are breaking the law we think that the authorities are most likely to turn a blind eye. The age of consent law is there to protect young people and clearly prosecuting a 15 year for under age sex will have the opposite effect. If you are in a relationship and you are under 16 but your boyfriend later turns 16 probably this will not be a problem providing you are similar ages. You should nonetheless be very wary about revealing the nature of your relationship to parents or teachers because sometimes parents will be extremely hostile and might go to the authorities simply to try to stop the relationship.