The VC10 has always had 3 altimeters. Plus another for the navigator and, in some marks, another for the air engineer.
To be compliant with RVSM requirements, the original main altimeter system was replaced with one of greater accuracy. Fleet compliance was granted for the C1K after a few HMU runs; however, every single VC10K had to do its own HMU run as the first few aircraft showed unacceptable scatter of results.
The programme also introduced the GPS/FMS/LINS system to meet MNPS requirements after Omega died.
Perhaps it was the downward extension of RVSM airspace, as well as its extension across most of the Northern Hemisphere, which caught the Hercules people by surprise?