is it bangkok you are going to and where from in the uk?
i have found an available flight heathrow - bangkok via moscow with only a short stopover in moscow both ways on the days you require via www.travelsupermarket.com with southall travel for £843.10 return inclusive of all taxes. ive had a quick look and there's not a lot of flights about and the ones that have availabilty are about £1200 - £1500
dept heathrow 21/12 12:25 arrive moscow 19:30, dept. moscow 20:45 arrive bangkok 09:10 22/12
dept bangkok 5/1 11:35 arrive moscow 18:05, dept moscow 18:55 arrive heathrow 19:45 5/1
outbound flight with transaero airlines, return with aeroflot.