I never understand why people create petitions (targeted at the gov) on a non-official site.

I'm not familiar with UK law, but what's the matter? They're equally valid in jurisdictions that I know of, a signature is a signature no matter where it was put

I'd personally just trust the government variant more with my government ID data than a third party but that's up to the petitioners to weigh and decide

In the UK, there's an official gov site for petitions, such that when a petition has >10k signatures, a government minister is required to write a response, and >100k triggers a parliamentary debate, iirc.

Whether the responses/parliamentary debates the person triggers end up being useful is up for debate.