Those are kind of last-minute flights, the cheapest fares currenly available with departures at the beginning of January 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 available already for Ł18 for double-room/night.
Booking samples
London - Mexico (Puerto Vallarta) for Ł271
Manchester - Mexico (Puerto Vallarta) for Ł285
Travel date samples
ex Manchester 1.-15. , 8.-15. or 8.-22.1.2015 (for Ł289)
ex London 3.1. - 17.1.2015
Booking sample of a roundtrip flight from UK to Mexico for Ł271 here. In this case you would be departing from London to Puerto Vallarta on 3rd of January and returning from Mexico back to UK on 17th of January 2015.