I did find ex AMS 29DEC - 26JAN in J for £1263.00 BUT with a few of draw backs ! This is not thru ba.com, only on Travelpack.com. Also, not same day connections either out or back, both need overnights in LON before carrrying on with the next. Also, this is on the direct LHR-BKK-LHR BA777 & from what I've heard the BKK route can get slightly neglected with poor cabins - Not to say you wouldn't get a perfectly decent seat / cabin and perfect flight !
If you are looking to spend £1K this could be an option ??