At TBIT you only have OZ RG and NH lounges as *G options, and from memory NH will be closed in the evening as is RG. OZ have a midnight flight 5 days a week but not sure if their lounge will be open as early as 1900/2000, which is when you need it for the 2100 SQ19.
Not sure if SQ are giving pax on SQ19 access to their lounge or not, given it's more of a prestige service flight.