Upvote:3
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.