FlyerTalk Evangelist
Join Date: Nov 2018
Location: Istanbul, Turkey
Programs: TK Elite Plus,BAEC GGL,ITA Executive, AFKL Gold,QR Gold,HH Diamond,Bonvoy Gold,ALL Gold
Posts: 14,186
You could technically do this, however, you would lose your rights for all the remaining flights in your ticket. FE: If you're returning to SIN on the same ticket that would be automatically canceled.
Also, there may be other consequences such as suspension of your FFP account, etc... but I personally haven't heard that much punishment from BA but heard from other airlines.