Remember it was suppose to be replaced by the apt in the 70's
APT was supposed to arrive in the late 80s perhaps into the early 90s, the APT-E only finished testing in 1976 so to produce the APT-S when the APT-P wasn't ready until 1981 (and even then it wasn't really ready until 1984) would have been impossible. I don't see how BR could have run the APT programme any faster than it did, remember APT was at the cutting edge of technology at the time and those sorts of complex projects take a lot of testing, development and most of all time.