(This isn't really a proper suggestion, just pointing out what's possible...)

If you are determined to avoid APD (one of the big charges on long-haul redemptions) you could consider positioning to JERsey and starting your 2-4-1 redemption from there?

You could book e.g. JER-LGW/LHR-SIN (with a <24h 'transit' through LON) plus SIN-LHR, and avoid APD on your booking.

This works even better if you happen to be able to tack a LGW-JER positioning flight on the end of another (earlier) redemption booking.

e.g. say you're already booked to go to ... err ... AMS, for the weekend. You book Reward Flight Saver for LHR-AMS, AMS-LHR, but you also add on a LGW-JER flight at a future date which fits for your 2-4-1 redemption. This effectively gets to you JER for nothing ('Domestic' connection on existing redemption booking), and your JER-LGW first leg of the 2-4-1 redemption doesn't incur any additional costs either - but it does avoid the APD.

This approach is nuts(!), of course, unless you like flying for no particular reason and/or you hate paying APD. This is flyertalk, though, so there is at least a chance you fall into one or both categories