I wasn't aware that following the rules of the air (ANO rule 17.5) was optional ...
True enough, but there is the matter of interpretation.
Plenty of scope here for arguing about what constitutes "vicinity", "pattern of traffic" and "ground signals otherwise indicate".
Flying has changed dramatically in the UK since that rule was originally drafted, for example the replacement of signal squares by radio comms, the use of non-standard patterns because of the threat of restriction/closure due to noise pollution, the growth in helicopter traffic, etc.
Operations that are routinely undertaken at a number of airfields around the country as a result of these changes would fall foul of a narrow interpretation of Rule 17(5). Fortunately, in most cases, a more pragmatic interpretation is taken!
To give but one example, how does a non-radio aeroplane legally undertake a right-hand circuit at an aerodrome that has no signal square?
At Leicester, I'm told you're for the high jump if you use anything but the strictest interpretation ... although they do still have an operational signal square!