What is the official state religion of the UK?

There isn't one. That's because there are four countries in the UK and they all have different approaches to religion. For example, in Northern Ireland and Wales there is no established religion at all, and hasn't been since 1869 and 1921 respectively. In Scotland, there is the Church of Scotland which is recognised as the 'national church' (and is in King Charles' coronation oath as such) but isn't a 'state religion.'

In England, there is an established church, linked to the government (e.g. 24 of its bishops sit in the House of Lords) - the Church of England - which is Christian and is considered the 'official' state church. But, it doesn't operate outside England itself.

So the official state religion of England is Christianity, but that doesn't apply to the UK as a whole.