The following Steam Payware is required for the route itself:
WCML Over Shap
Edinburgh to Glasgow
London Faversham High Speed
West Highland Line South (from Phase 6 onwards)
West Highland Line Extension (from Phase 6 onwards)
Glasgow Airport Rail Link (from Phase 6 onwards)
...plus the UKTS Freeware packs related to scenery, which can all be downloaded (free) from here.
Note, scenario prerequisite information has now moved to the Scenarios section.