Those are kind of super last-minute flights, the cheapest fares are currenly available with departure already tomorrow 25th of April 2015. Your return flight from Mexico back to UK would be then either one or two weeks later. 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 available already for Ł16 for double-room/night.
Booking samples
London - Mexico (Puerto Vallarta) for £249
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 £249 here. In this case you would be departing from London to Puerto Vallarta on 25th of April nd returning from Mexico back to UK on 2nd of May 2015.