[iso1631]

Still the case long haul

Just looked up economy London-Singapore return, Sunday-Sunday in economy on BA Aug 7 to Aug 13

and there's space in the 'M' bucket with fare MHNC80S7 which is £510 each eay (plus fees and taxes)

  TRAVEL FROM LAST INTERNATIONAL STOPOVER MUST COMMENCE NO
  EARLIER THAN THE FIRST SUN AFTER DEPARTURE OF THE FIRST
  INTERNATIONAL SECTOR.

Come back a day earlier though, Sunday-Saturday, and you can't use that fare. The cheapest they offer is in the "Y" bucket, YNNC80S1, at £1131.50 each way

That means the extra day in Singapore saves you £1200. (the taxes and fees are the same regardless)

If you travel a lot - especially flexibly - then it is sometimes worth a subscription to expertflyer, which if far faster at seeing rules and availability than traipsing through ITA Matrix.