Sale fare in Club Heathrow to Narita was £2195, but currently running at £3250 (!).
You might be better off taking advantage of the exEU sale which runs until Wednesday 12th June. Narita from €1969, around £1675 plus the cost of getting to and from AMS (for which the Saver redemptions are perfect):
Taste of London usually triggers some sort of offer/money off code so keep an eye out for that, thereafter there is usually a sale in late August.
Book as a BA Holiday for a 10,000 avios bonus!