Your examples only serve to show what a shithole nanny state the UK has been sliding down towards to, and what a slippery slope all these "omg think of the children! 😱" legislations are.

The alcohol you have in your home you had to be legal age to buy in the first place. Similarly if you had a porn DVD at home you would have had to prove your age when you bought it

I live in a EU country, never in my life have I had to provide an ID to buy alcohol or pornography, neither online nor in person.

Why is online special?

Online, there are risks of privacy and security. It's already difficult enough to maintain a reasonable security and privacy stance that balances between convenience and not being tracked and targeted everywhere, without putting age gates into the mix. Even if you made the perfect age gate app without vulnerabilities (which you can't), that perfect app could still be spoofed to trick people into providing sensitive identifying information to bad actors. It happens with banking apps, it will happen with age gate apps.

In real life the government does not get in the middle. It is a private transaction between a buyer and the seller, and the unspoken assumption is that the buyer is an adult of legal age. Only when there are serious doubts about the buyer's age will the seller scrutinize. Online, the assumption of being bona fide is reversed: the assumption is that everyone is a minor until proven otherwise.

Online is also typically not a one stop transaction. In a single browsing session an adult might want to access many different pieces of content, spread out over several different sites. Adults having to stop and prove their age at every turn online is burdensome, draconian and has a huge chilling effect. Data has shown that sites that introduce an age gate, only retain about 10% of their users. So the other 90% either goes dark or is dissuaded entirely from accessing said materials. Neither of those are good outcomes.

Online is also special in that it doesn't even work. An online age gate doesn't really prevent anything, it just sends traffic elsewhere, making it little more than a nuissance. If a liquor store denies a minor buying liquor, the minor is SOL because there are only so many places they can physically try. Online they can just click the next link, or the next, or the next,... It's simply impossible to age gate all the sites where you can find porn. And yes, it's ridiculously trivial to find non-age gated porn, when I tried it with a UK VPN yesterday it was as simple as typing "porn videos" in DDG, and clicking the first link.

Finally, there is also a huge difference in harmfulness between consuming certain physical substances like alcohol, and viewing adult content. The very idea that it is particularly harmful for teens to view sexual materials is scientifically dubious, making this more an overbearing and disproportional "moral panic" type of reaction than a proportional, well studied and well reasoned measure. It also conveniently ignores and does nothing about much greater harms that young people fall prey to online, like what TikTok is doing to the attention span of kids, or incel/manosphere echo chambers and various other misinformation spheres, or online bullying, or screen addiction, or unrealistic and ultra-materialistic world views promoted by influencers. It aims to be a technological solution to a tiny part of a much larger societal problem, and that never works.

In my opinion, the true intent of this legislation has never been to protect children. Instead it is a power grab by a control obsessed government, and an ideological attack against those who create, distribute and view porn. The children, as usual, are only there to provide emotional blackmail to get people to accept intrusive, draconian measures. And you, my friend, fell for it hook, line and sinker.