I checked LON-BKK on EVA's website for 4/3-4/17.
ECO Special is 627 GBP
Elite saver is 825 GBP
Business saver is 1,900 GBP
LON-BKK is 1,040 or 1,328 GBP, depending upon the day
BKK-LON is 859 or 1,146 GBP depending upon the day.
If you have any flexibility in your schedule, you might be able to save a little.