IMHO:
1. If ATC want 180K, and 'S' is say 183K, F1 will give you lower fuel flow and acceptable. As you get further below 'S' I get uncomfortable with the high nose attitude. However, the min legal speed is VLS...
2. Getting near VLS in this scenario is going a bit too far - you cannot use S/B without it powering up against you etc.
I don't believe BA's SOPs are 100% prescriptive in this area, and that's probably the best way. Often I will fly a bit under 'S' @ 180K F1 but as the G/S approaches take F2 to use the drag down the slope (F1 it will accelerate), and avoid the F2 "balloon" while trying to hold the G/S...