The absolute lowest price for J I have seen LHR-BKK in the last couple of years is 1.7k GBP - the current best price for saver J comes in at around 1.9-2.1k at the moment. If you're already seeing 4.3k and then 6.5k for dates next to each other, I think it's unlikely that the 6.5k price will come down as the price has most likely moved to flex buckets due to strong loads.
Have you considered the 2215 departure from LHR on Friday? It would help if you could give us exact dates/times for some better advice.
EDIT: I assume you are looking at 20/21 March 2015 - Expertflyer shows quite open availability for the Saturday ex-LON departures, so this would be one of the rare occasions where I would say wait a bit before checking again as new fares might be loaded a bit closer to the time. Either that or go on Friday...!
Last edited by eternaltransit; Jun 22, 2014 at 9:15 am