Flying to Cancún
From London: British Airways has direct flights to Cancún on Mondays, Wednesdays and Saturdays. They also offer good flight + hotel packages. Be sure to check the hotel you choose is in Tulum or close by.
From Canada: West Jet has direct flights to Cancún on Wednesdays and Fridays (don't know if sunday and monday, too). We also recommend to check for flight + hotel packages, in one hotel in Tulum or close by.
Citizens of European Countries, Canada, USA, Australia and many others, don't need a Visa to enter México as a tourist. Same case if you don’t hold a passport from one of the countries on the ‘exempt’ list, but you do hold a permanent residency visa or a current, multiple-entry visitor visa for the United States of America, Canada, Japan, the United Kingdom, or any country that is signatory to the European Schengen Area. More info HERE.
From Mexico City: our favorite option is Interjet. They have a lot of flights daily to Cancún.
Remember the sooner you buy your ticket, the cheaper it will be, since prices rise on demand and availability.