Yeah, but, you try a web search for the UK rules! - you just get dozens of links of the form given in the OP. And one has the impression that it's been changing every five minutes for the last several months, and you've already been told that the text books are out of date, so how on earth do you think the instructors could find out what the score was to get it right in the "theory lessons"? - and even if they did, that was last week, who's to know the rules haven't changed again this week?
You can see why some people think "sod it, I'll just go flying".
This does not apply for the UK alone. We all know that regulations reached a state of nonsense for pilots, so we have to get used to it, because we are not allowed to shoot the theory clerks in their armchairs.
I expect every student in pilot training to gather some sort of intelligence how to maneuver through the mud of regulations and documents. Searching the Internet and using Google, not as a brain-extention but brain-replacement, is not on that list of skills. I expect a student when ready, to know the methods to imagine and track the way to where the information I need is found. As this is highly local business, it is or should usually be taught during theory lessons.
For the semi-circular rules, as this is a practical maze example: It is not set in the European rules, but choice of the local country, so it has to be in some local regulations. If I remember correctly, in Germany the basic regulations are in local LuftVO §31 and §28 and the setting of 5000ft MSL or 2000ft AGL, whichever is higher, for use of semi-circular rules are in local "Nachrichten für Luftfahrer" NfL I 190/01. In France you have to go to
SIA - La référence en information aéronautique and search for the 3000ft MSL/1000ft AGL setting for semi-circular rules. As far as I am aware, the UK has not set special rules for circular rules beyond TL at 3000ft and at some places 6000ft (AIP ENR 1.7-1Kap4). But UK chose to recommend VFR taking the same IFR altitudes, wich feels kind of odd for continental flyers. Actual setting are not in some unqualified internet databases, but in the official documents, to start with the AIP.
It is a pity we do not have a single and open data base, maybe housed at Eurocontrol for Europe, due to local small kingdoms, but thats the way it is today.