> Britain [has] less interference in policy from religious extremists [than the US]
This seems a slightly rose-tinted view of British history. I would argue Britain actually has more interference in policy from religious extremists than the US. What would you call the Lords Spiritual (the 26 seats in the House of Lords reserved for Anglican Bishops) if not religious interference in policy?
Or how about Section 28, which was proposed and pushed through primarily by religious groups? US government policy is frequently influenced by religion, but the separation of Church and State tempers this, and the Supreme Court has frequently overturned legislation that has been drawn up with religious influence (and indeed, directly overturned legislation that actively promotes religion in state institutions). In the UK the church is the state.
You only have to look at the recent issues surrounding gay marriage (versus civil partnerships) in the UK to see there is religious interference in governmental policy.
> a history of judges making activist decisions in order to preserve civil liberties where legislation threatens to undermine established convention
I would argue the US has an equally strong history of this, and the current Supreme Court system in the UK owes a lot to the US. Indeed, I think the etymology of the phrase 'activist judge' stems from the US, not the UK.