score:7
It's Option 1: the 6 months is renewed every time you enter.
However, as you say that would be very easy to exploit, so it's explicitly not allowed to live in the UK by exiting for a few days every 6 months.
From Gov.uk (this quote is about visitor visas, but as a visa-free non-EEA visitor the same rules apply):
Your visa may be cancelled and you may get a long-term ban on visiting if your travel history shows youβre repeatedly living in the UK for extended periods.
The obvious question then is how much you can get away with, and there's no hard and fast rule, but this answer is about as good as it's possible to get.