Not a mock-up, it was for real, for about a year in the late 70s.
Joint BA-SQ service LHR-BAH-SIN. One side of the a/c was painted in full SQ colours. Discontinued officially due to lack of supersonic overflying rights over Malaysia, but suspect low yields and high oil prices had something to do with it.
Sad, sad day today...
http://www.flyertalk.com/forum/Forum15/HTML/003673.html
Perhaps, after all its glossy advertising talk about sonic cruisers (circa 1990), SIA would like to put its money where its mouth is, and restart services again? Surely that would be a way to get 5th freedom LHR-JFK rights...