This might sound a little crazy. Will SQ allow me to fly into Narita, and then fly back on the same day from Narita? Thinking of a SIN-NRT (one way), and NRT-SIN-LHR-SIN-NRT (down the road assuming Japan opens up...).

I don't suppose I'll need a visa if SQ allows. Alternatively, does flying into Narita on ANA for the SIN-NRT leg make it easier?