Thailand as one of the most demanding destination during European winter is still possible to visit during running main season for a budget price thanks to Jet Airways! (3-star airline by SKYTRAX). Currently you may book very cheap open jaw flights on route Brussels to Bangkok and returning back from Thailand to London. (open jaw flight means that you will book multi city ticket where your return flight will be departing from or arriving to different airport than the original one). Air ticket on said connection are available for just Ł354 which is roughtly €423!
There is plenty of travel dates available even during main tourist season in Thailand for price at this rate level. Tariff of Jet Airways allows to book these multi city flights till the end of May 2014 (departure date from Europe). There is just one short layover in India on your way to Bangkok.
Brussels and London are connected by regular carriers (Brussels Airlines, British Airways) offering one way flights from Ł60. If you want to travel on the lowest budget possible you may use service of couch provider Megabus. In such case you would pay for one-way ticket just Ł12.
In case of interest you may book the tickets through booking agents Ebookers. or Expedia.
Thanks to extremly high competition in Thailand you may find all the kind of service for really cheap price. On the other hand Bangkok as well as whole Thailand offers also super luxurious 5* hotel chains. It is just up to you once you arrive there which service you choose but the fact is that Thailand still remains one of the most rewarding destinations for casual tourism. This is indeed one of the last possibilities to book cheap flights to Thailand during actual main tourist season for such a reasonable price!
Booking sample of a open jaw flight for Ł354 departing on 5th of February 2014 from Brussels to Bangkok and returning back to London from Asia on 19th of February 2014: