Quote from Foreign & Commonwealth Office advice
ENTRY REQUIREMENTS
British tourists visiting Mexico need a valid passport and should obtain a tourist card (known as the "FMT") available from Mexican Consulates, Mexican international airports and border crossing points and most airlines serving Mexico.
Following the March 2004 incident with UK cavers, all visitors who are uncertain about their visa requirements should check carefully with the Mexican Embassy in the UK, and request written confirmation if necessary, Mexican Representation in London.
The activities permitted under a tourist visa are not clear. In particular, those planning to participate in adventure tourism, or activities requiring special equipment or involving visiting isolated parts of Mexico, should clarify their visa requirements in writing before their visit. For example, caving and potholing have been classified by the Mexican authorities as a scientific activity, not a recreation or sport. The status of other adventure sports and eco-tourism in general (eg bird watching, entomology) is also unclear.
Military personnel or civilians with military connections, whatever the purpose of their trip, should record this in writing to the Mexican authorities when seeking visa advice.
If you are under 18 and are not travelling with both your parents, special rules apply. If you are travelling with only one parent, you must be able to produce notarised consent from the other. If you are travelling without either parent, you must bring notarised permission from both. If in any doubt, consult the Mexican Embassy.
You should not bring meat or dairy products to Mexico. The importation of these products from the EU is prohibited.
see the FCO site
http://www.fco.gov.uk/servlet/Front?pagename=OpenMarket%2FXc el