The 166 can only be used on Brighton and Great Western Express in timetable mode. LBR could be Brighton, but I think the more generally accepted code is BML (Brighton Main Line).
Not sure if they make it as far as west Cornwall, there's a few 16x based at Exeter but not sure what their range is beyond the Devon Metro
I do remember seeing photo of the class at st ives (where wcl is set) so they can be layered there however it would require a castle hst, IET, 158 etc all in gwr colors and a new timetable just to be added.
Scenario planner allows for the GWR stock WITHOUT 'Off the Rails' enabled... but apart from London Commuter there are no other routes to drive the 166 on