There's a difference between the situation where a country insists that certain agreed customs and rules should apply to anyone who comes to live there, regardless of their previous traditions, and where it seeks to impose its own ways on another country.

The former is basically straightforward in theory though tricky enough in practice. The latter is more complicated, because that kind of imposed change tends to backfire.

With Pitcairn Island you have something which falls in between, because, from all I've heard, it doesn't seem to be a viable "country", and is heavily dependent on New Zealand. (NB "British Law" in this context doesn't mean Britain, it means New Zealand, with a system of law derived from Britain.)

In a sense it's analogous to the pre-Civil war USA, where part of a country has a peculiar way of life which involves behaviour which is seen the rest of the country as evil.