Hi and a warm welcome
You will be awarded tier points per individual flight segment. So for SQ flights over 2000 miles, in economy class, you will receive 2 tier points and for SQ flights under 2000 miles you will receive 1 tier point. For a round-trip LHR-HKG via SIN you will receive 6 tier points in total, enabling you to reach Flying Club Silver.
You will also earn approx 4500 miles more than if taking the direct VS flights. As you will probably be aware, the cheapest VS fares (N,X,Q, and V classes) to HKG will accrue only 50% mileage from next month.