As I mentioned in another comment, I've heard no compelling argument that differentiates between this scenario (e.g. kid uses this site to access a nsfw subreddit) and an underage kid buying smokes with a fake ID.
In that scenario, the police don't just pick 1 entity to punish. The kid gets in trouble, the store (most likely) gets in trouble, and (if found) the fake ID supplier gets in trouble.
In the end, I hope that the owner of the website never has to find out exactly where and how the laws shake out, and that "it's a satirical website" is a strong enough defense. But from my armchair, I would suspect that the UK police/legislators would not look favorably on the "it's satire" defense. Especially because of this post which advertises that the fake ID works for some services, and there are under-18s on HN.
>Law often focus on intent.
I would expect that advertising that the IDs work on real services undermines any defense of "my intent was satire".