Such examples can be found all over UK law, it is, as you say in effect a by-product of the common law system.
But also it also comes from the way legislation is written in the UK...
1. A member of parliament (MP) brings a "bill" to Parliament proposing a new law or a chnge in the law.
2. If it has legs, then it will end up being debated in Parliament
3. Almost always politics starts coming into play and MPs of all parties will usually propose real or party-political amendements to the bill which are then debated and voted on.
4. You then have a ping-pong process with the upper house (Lords), stuff goes back and forth between Lords and Commons ... more amendements are made along the way.
5. Finally once everyone has come to some sort of agreement it goes to the monarch to be signed into law (this is purely ceremonial, the document is presented to the monarch as a done-deal).
So that's why you end up with all these messy looking and almost incomprehensible pieces of legislation, there's quite a lot of "too many chefs" going on at every stage apart from the first and last.
Therefore as you point out it looking at UK law in the absence of case law is a fool's errand.
It is also where good barristers are worth their money because they are know the case-law like the back of their hand, and if there is no case law for your situation, they will have a good handle on the "letter and spirit" of the law in question and hence your prospects if it goes to court.
Most important legislation, including the BNA, is government legislation (indeed, see the white paper: https://www.uniset.ca/naty/maternity/wpaper.pdf). It is therefore drafted by parliamentary counsel, whose advice remains available when amendments are proposed. Most governments also command sufficient majorities to push this kind of legislation through, or at least to come to consensus on amendments. The relevant passage seems clear enough that parliamentary counsel could have drafted it and so I doubt there were ‘too many chefs’ as you put it (although I haven’t checked Hansard).
It is also hard to see what these drafting habits have to do with the common law system. Points 1–5 could be true of a legislature in pretty much any legal tradition.
Moreover, the HoL and the "ping pong" process actually gives the legislation a chance to be refined, as the lords are less motivated by party politics and more able to focus on getting good legislation through. I've heard a number of complaints about the current system that basically say that without the HoL, the quality of legislation would be significantly poorer, and that there needs to be more work done on getting bills written properly in the first place when they get proposed in the House of Commons.
But also it also comes from the way legislation is written in the UK...
So that's why you end up with all these messy looking and almost incomprehensible pieces of legislation, there's quite a lot of "too many chefs" going on at every stage apart from the first and last.Therefore as you point out it looking at UK law in the absence of case law is a fool's errand.
It is also where good barristers are worth their money because they are know the case-law like the back of their hand, and if there is no case law for your situation, they will have a good handle on the "letter and spirit" of the law in question and hence your prospects if it goes to court.