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.

> "unless the contrary is shown". Shown by whom? To what standard? What must be shown?

I have no idea about British law, but... As I get it, here we see the fallback logic that defines some defaults when dealing with lack of information (an abandoned newborn). Here we don't know newborn's nationality, so we must make an assumption - the lawmakers decided they didn't want to have proverbial NULLs in here.

- "What must be shown?" - I could be misunderstanding things (especially because English is not my native language), but I believe the intent here is pretty clear. To me it reads as applicability scope, constraining when the fallback kicks in, so we don't create logical contradictions. My understanding is that it says that if there's a known fact that contradicts (a) or (b) it would make those defaults inapplicable - which is logical, because we can't proceed to assume A if we know that ¬A is true. E.g. if there's a known fact (say, a DNA test or a witness testimony or some other evidence) that confirms e.g. that the infant's parent wasn't and isn't a British citizen or permanent resident, this piece won't apply and we won't consider infant to have been born of such father. I think the program at the bottom of the page 374 (page 5 in the PDF) agrees with me on this.

- "Shown by whom?" - as this is clearly unspecified, can't we safely assume that the existence of the fact alone is what should affect the execution flow, not who brought this fact to the light? That is, I believe it reads so anyone or anything can show it, but only the fact (and its truthfulness) itself matters. Again, don't see the issue here - but I'm curious if you have alternative interpretations how it could be read?

- "To what standard?" - this is the only real ambiguity here. Probably because the lawmakers writing the act weren't exactly capable or even aiming to figure out the theory of truth for a natural language. As I get it, for the legal stuff it's pretty normal to have undefined behaviors and return to them when they are triggered and someone is unhappy about the outcome and goes to the court to follow up and clarify the details. When we're rendering laws as machine programs, we mustn't forget what they apply to - to the society governed by those laws. Unless we're running a simulation, the society is external to the program - and it's the only sane design (that doesn't violate Kant's categorical imperative, so the laws should not treat the society as means to some goal but as the goal itself - if I understand the idea correctly, please correct me if I don't) that any or almost any statement may raise an exception-like situation that would require a human to look into it and fix (thus always deferring ultimate legislating to a rational being's will). In other words, it's our usual "I don't want to think too much about this edge case, gotta ship this thing already, so we'll get to this later if we get an error report". This is the universal mechanism that applies not just to this particular question, but to the other questions above that I thought are clear, if someone disagrees.