International law limits state jurisdiction to territorial boundaries (Art. 2(1) UN Charter). Hacker News is a US web site and Y Combinator LLC is a US company. The OSA, which is a UK law, cannot mandate physical enforcement (e.g., server seizures) on foreign soil. If they really didn't like HN, UK government could try to suppress HN access for their citicens by local means. If HN had a branch in the UK, the UK government could take action against that branch. As far as I know that's not the case.

Yes, but I don't really understand how the UK can expect to enforce this law against non-UK entities that don't have any employees or physical presence in the UK.

HN/YC could just tell them to go pound sand, no? (Assuming YC doesn't have any operations in the UK; I have no idea.)

they could impound you while on a layover in UK (not that you'd ever want to do that)

a good reason never to visit the UK again

They could open an international arrest warrant. So you can't travel at all.

pg lives in Britain if I'm not mistaken.

Yes.