My boyfriend will buy two return tickets in Mexico to travel to the UK. One for him to travel to the UK and back to Mexico, and one for me (I am already in the UK) to travel to Mexico. Is it possible for me to use the return ticket that he buys me as a one way, or am I not allowed to do that because I haven't actually come from Mexico to the UK and back to Mexico again? Am I able to travel to Mexico from the UK (on the return ticket that my boyfriend bought for me, using it as a one way ticket) if I haven't actually flown from Mexico on it?
As a general rule, you will not be able to travel on the return ticket without taking the outgoing flight. Pretty much all airlines will cancel the entire ticket if you miss the first leg. Some airlines may make exceptions, but it's very rare and is considered on a case by case basis and is usually allowed in exceptional circumstances. You really do need to buy a one-way ticket from UK to Mexico.
The only notable exception to this rule is the no-frills budget airlines, which don't really sell return tickets but rather a pair of one-way tickets.