Thanks to current offer of Oman Air (4-star airline by Skytrax) we may book an open-jaw flights to Thailand for quite interesting price! (even at the end of running tourist season) Open-jaw flight means that your return flight will depart from or arrive to different airport than the original one. In this case you have to look for connection with departing either from Paris or Zurich to Bangkok. Your return flight is then back to London Heathrow. If you are planning to fly to Bangkok you should consider this offer..
Both Paris and Zurich are connected with London with many low-cost carriers. One way tickets with Easyjet are available from Ł36 (hand luggage included only). If you would prefer classic airline you can book with British Airways for Ł61 (including check in luggage). The easiest way how to check best connection flights within European cities is to use Skyscanner.
How to book
Booking has to be done till 15th of January directly with Oman Air. (or for a bit higher rate through Ebookers however it seems they offer more travel dates available). Look for the travel period between March and June 2014 if you want to purchase the tickets at this low airfare price.
Booking sample of an open jaw flight for Ł345 (€417). Departing from Paris on 12th of March to Bangkok and returning back from Thailand on 27th of March 2014 to London Heathrow: