yep they use it for everything and is the defacto flightnumber. Its all offical through the agency that deals with callsigns.
And it really screws with the system apparently when you have two mode S with the same flight ID on apparently.
And quoting PAN -ops and ICAO never works with UK ATC system there are getting on for 9 pages of differences in Jepps. If they published the VFR differences as well it would be bigger than the all the other countrys differences added together.
And its your responsability as PIC to set the aircraft up for your flight what ever the condition it was left in by either the engineers or previous pilot. You should also be current with the operation of the equipement onboard.