The help pages are something I have recently had words about. Not too long ago they "simplified" them and as a result took out some of the stuff.
Believe it or not I have been unable to find anywhere on CAA or AIS an explanation of what is and is not valid in the route box of an ICAO Flight PLan. The Safety Sense leaflet on VFR Flight Plans doesn't tell you. It used to be in the AIS help file but last time I looked it had disappeared.
There has been quite a lot of effort to help educate users. There have been articles in Flyer and in General Aviation, the AOPA magazine. Pilot published a users guide and the next edition of Popular Flying, the PFA magazine will also carry an article on how to get the best from the site.
In adidtion to this Pilot sponsored me to be on their stand at Fly!, Aerofair and the PFA Rally last year to help and demonstrate how to use the site and I have also presented at West London Aero Club, in central London, and at AIS during user visits.
AIS are happy to have visitors and if a group from your flying club want to go, just speak to them. They are very approachable, as you have found.
Mike