I don't grasp the boldened sentences. If a member of the House of Lords last held office as Prime Minister in 1902, then how can't we "conclude that this has always been the case since 1902"? Doesn't "such an occurrence is impossible today" prove this has always been the case since 1902?
Pls see the question in the title. To tell whether practice on a certain matter has hardened into a rule, don't you just regard the duration of the practice? 1708 was the last time a monarch (Queen Anne) refused assent. 1965 was the last time a non-member of either House of Parliament entered the Commons at an early by-election, "since no government today willingly causes a by-election to be held in one of its own seats" (p 21). Thus royal assent has lasted longer and is more of a rule than the principle of ministerial responsibility that a minister belong to a House of Parliament.
So what if "the practice is negative in character"? You can frame the above two Constitutional Conventions as negative practices:
"Queen may not of her own initiative refuse the royal assent" to a Bill passed by both Houses of Parliament.
"There is no rule of law which prevents the monarch appointing to ministerial office a person who is outside Parliament." (p 21)
Bradley, Ewing. Constitutional and Administrative Law (2018 17 ed). p 23.
The starting point for whether or not a convention exists is usually the tripartite test proposed by Sir Ivor Jennings: ‘first, what are the precedents; secondly, did the actors in the precedents believe that they were bound by a rule; and thirdly, is there a reason for the rule?’139 This approach is, of course, contested; certainly all of the stages of the test contain important ambiguities or disguise value judgments.140 It is highly unlikely that all conventions require all limbs of the test to be satisfied, at least not in the same way or to the same extent: the Sewel convention noted above, for example, came into being effectively immediately without the need for historic precedent.141 Other long-established conventional rules have great authority and are universally known. Many have developed out of a desire to avoid the formality, explicitness and publicity associated with changes in the law. The role of the monarch in the conduct of government has almost disappeared since the 18th century without a series of statutes removing one royal power after another. In the same way, powers have been acquired by the Prime Minister by operation of convention rather than as the result of legislation. Conventional rules may be used for discreetly managing the internal relationships of government while the outward legal form is left intact.
The informality of such rules is often accentuated by the fact that the rules themselves are not formulated in writing, but this is not always the case. As we have seen, the rule that judges should not undertake political activities is now in a written form. In 2011, the government
issued The Cabinet Manual, which derived from a similar document prepared previously in New Zealand, as ‘a guide to laws, customs and rules on the operation of government’.142 The booklet covers all the machinery of central government in an informative way, but it is essentially descriptive; it does not claim any special authority (it has not been approved by Parliament), and the government reserves the right to change the practices that are described. In reality, some of the practices are too well-established to be changed (for instance, the rule that the Queen’s speech in opening a session of Parliament is written by the government).
The development of unwritten rules is often an evolutionary process that occurs before clear rules of conduct emerge. In retrospect, we can identify when (for instance) a member of the House of Lords last held office as Prime Minister (1902). But because such an occurrence is impossible today, we cannot conclude that this has always been the case since 1902. At any given time, it may be difficult to tell whether practice on a certain matter has hardened into a rule, particularly when the practice is negative in character. Before the Constitutional Reform Act 2005, which took away the judicial role of the Lord Chancellor, there was uncertainty over the extent to which the Lord Chancellor, as a government minister, might properly sit as a judge to decide appeals in the House of Lords.143
As with all forms of rules, disputes may arise about the meaning and effect of conventional rules, particularly when they have no definitive written form. The enforcement of many conventions depends essentially on the force of public and political opinion. If many legal rules have an ‘open texture’,144 how much more ‘open’ will be the texture of non-legal rules where there is no definite procedure for resolving disputes about their existence and content. The vaguer the convention – perhaps better expressed as a principle than a rule – the harder it will be to show consensus as to its boundaries, unless there is a single authoritative figure who can enforce it (such as the Prime Minister with the Ministerial Code).