Why do you expect to find guides to fly through other bits of UK airspace?
Well only because it would make life easier for everyone, and it would also be a major safety advantage to have pilots understand the airspace that they are or will be flying through. If all pilots understood the airspace, understand the services provided, understand the boundaries and limitations of services and/or airspace in complex/congested airspace. It would also make people who are not familiar with the airspace more confident of what they should expect, it would make general aviation in complex airspace more accessible to all.
Having detailed / photographed instructions for using various "corridors" around the London TMA for instance would also greatly reduce the number of airspace incursions, and pilots would spend more time looking out for one another rather than figuring out how close they are to class A or D airspace.
This would, at least in my mind, open a lot of possibilities to both new and old pilots who fear the more complicated airspace. The few I met who have let their licences / ratings lapse has been due to the lack of excitement, fear of exploring further afield, and the obvious lack of funds. Surprisingly however, the former 2 seemed quite prominent in various airfields I have been to. I do believe that offering more humanly understandable information would definitely help keep general aviation more accessible.
They chose to put it on the Fly-on-Track website, which has a wide audience.
You say they chose to publish there - why would they choose to publish their guide on a third party's website? I don't understand the logic... Don't you wish to have a full resource of the ATS that NATS provides general aviation in a single place, owned by NATS so that users are sure that they have the latest version of the guide?
FAA website today reads: "We are currently processing permanent Airmen Certificates for temporary certificates that were issued approximately November 15, 2016." Yes, I know you can fly on the temporary certificate. That would solve a lot of problem if adopted by the CAA.
I agree with that statement entirely, the UK CAA allowed some completion certificates to be issued but not for initial licence issue (I believe).