First of all, SQ to LHR is very crowded especially during holiday period. F, C and Y cabin almost full. Even for revenue pax, they need to get waitlisted and even cannot clear. Happened to me once. Do you have any status with SQ?
The chance is very slim I think and secondly, I do not understand if you need a sudden trip back to London, why do you still need a redemption booking instead paying for C cabin if it is an emergency. I dont think someone who need to go back immediately for an urgent case still thinking about redemption ticket. Your statement about sudden trip is a bit odd and obsolete as you still think for redemption in order to get back to London immediately.