This type of information often surfaces publicly (unofficially) through driver training materials that find their way into enthusiast groups. The SimSig documentation is often a reliable source for older interlockings where the simulation remains mostly current, sometimes there's source documentation as well. In the case of Salisbury, an old signalling plan is in the further info section.
https://www.simsig.co.uk/Wiki/Show?page=simulations:salisbury
Salisbury station area itself is 3 aspect, with all lines approaching being 2 aspect with distant signals (except for the odd 3-aspect one protecting sidings).
None of this should be used for "official" work, as proper sources should be obtained, however it's often a good enough pointer for unofficial projects.
Regards,
Thomas Wood