His father doesn't like the idea of sending the original of his UK birth certificate - why they can't at least accept certified or notarised copies is beyond me.
Nobody has the original of their own UK birth certificate. For births since 1837, the "original" is part of the birth register held by the council where the birth took place. Anything you hold is a certified copy (including the certified copy that you purchased at the time you registered your child's birth).
Since it's a public record, anybody can buy anybody's (including deceased people's) England or Wales birth certificate for £12.50, while it costs £15 for births in Scotland and NI.
The rules around automatic citizenship by descent are antiquated and imperialistic, assuming that a country is superior and therefore everyone wants to be a part of it. Sure, give people an option to take up the "right" to citizenship - which is what we had always assumed MrJones had.
With the new ETA rules for "citizens" it seems clear that the UK government haven't really thought through the issue of the citizens by descent, and the ludicrous situation they have caused.
Seems this UK thing is similar with the UK giving out automatic citizenship to people who never wanted it or tried to apply for it.
A British citizen born in the UK moves to China and has a child. From birth, the child is automatically a British citizen and allowed to live in the UK. Not sure how this is antiquated, imperialistic or ludicrous.
As long as the child has their own birth certificate and knows their British parent's name/DOB, they can order a copy of their parent's birth certificate to apply for a passport many years later.
However, if the British citizen parent wasn't born in the UK and didn't obtain their UK citizenship by naturalisation, the child would (in most cases) need to move to the UK for 3 years before being eligible to become a citizen. So in this case if you as the parent didn't want your child to become a citizen, you could just not let them move to the UK until it's too late.
Conversely, if an Australian citizen moves to China and has a child, the child will only become an Australian citizen if someone takes positive action (and pays the fee) for them to become a citizen, and then they also need to apply for a passport.
Both approaches have pros and cons.
In fact, China's approach to descent may make sense to some people - when a Chinese citizen has a child born outside China, the child only becomes a Chinese citizen automatically when the parent is temporarily in that country. If the parent has permanent residence, the child won't and can't become a Chinese citizen, unless the child can't get any other citizenship. I raise the example of China because I find it funny that it will soon be easier for Brits and Australians to enter China for tourism than to enter each other's countries.
I didn't think that eligibility for citizenship translates to 'actual citizenship' until it is requested and applied for and all paperwork approved amd recorded. If you haven't applied for it then it hasn't happened.
There are two scenarios, which I already described in #
125. Depending on personal circumstances, some people are automatically British citizens from birth, while others need to register or naturalise first under a simplified process before they become citizens. The issue in this thread only affects those who are automatic citizens.
So, if this was automatically bestowed, this is citizenship by descent then? So Dad was a Brit and maybe never knew it.

Although im trying to find out if he registered citizenship in his own right and then I can claim it too. His parents migrated to Aus in 1926. 100 years ago. His passport issued in 1948? Has the British Subect stamped on it.
Australians citizens were British subjects until 1949, then generally became Citizens of the UK and Colonies until 1982.
Based on what you've said, if your dad wasn't born in the UK then you won't be a British citizen.