Apart of the classic beach destination Cancun you may also consider to visit Pacific side of Mexico - Puerto Vallarta. Based on service of charger provider Thomson Airways you may book super last-minute flights to this destination from just £239 (€331)! The lowest fare is possible to reach when departing from London or Manchester. This fare is for a return ticket including all taxes and fees except exit tax approx 900 Mexican Pesos payable on departure..
Those are kind of last-minute flights, the cheapest fares are currenly available at the end of October 2015. If you would prefer Caribbean side you may consider to book your flights to Cancun when departing from UK (London Gatwick). Accomodation in Puerto Vallarta in Mexico is possible book via Hotelopia or Venere.
If you consider to book this deal you may find some cheap connection flights from your hometown using service of Skyscanner.This tool helps you quickly to find out which low-cost airline is connecting from your city.
Booking sample of a roundtrip flight from UK to Mexico for £239 here.. In this case you would be departing from London to Puerto Vallarta on 30th of October 2015 nd returning from Mexico back to UK on 12th of November 2015.