SQ and BA do both use Terminal 1; however, they use different satellites at that terminal. Because you need a valid boarding pass to reach the satellite, you would not be able to meet your companions as they deplane. (Also, maybe the EuroCrowd can let you know if it's a different customs procedure arriving from UK than a non-EU country.)

I believe there is a point de rencontre (meeting point) in T1, though, or you could agree to meet at any of the exit doors (numbered 1 through about 32).