On your SEA-LHR leg you will have access to Galleries Arrivals in LHR T5 (if you go landside), and the Concorde Breakfast Room within this lounge as you are in F. There is an Elemis Spa in Galleries Arrivals. Galleries Arrivals is open 05:00-14:00.

On your LHR-SIN leg you you will be departing from T4, where the only facility left open is the BA Gate 1 Lounge. There is a pre-flight dining area for F passengers, but otherwise services are the same as for J passengers. Note that all the other T4 lounges have closed, including the Elemis Spa.

You do not have access to any of the T5 departures lounges.

Your best option is to use Galleries Arrivals for a couple of ours which will allow you to indulge in a massage or two. I wouldn't really want to spend the next 7 hours in T4 with the reduced service now offered by BA out of there, but it won't be too bad. The BA Gate 1 Lounge will likely be quite quiet until it fills up later in the evening with the Aus flights.