In saying that the UK's adoption of part of Sharia law is inevitable, I suspect the Archbishop of Canterbury is making the same mistake he made in calling for laws against "cruel speech." He's failing to see that there should be a (big) space between individuals and the law, a space filled by civil society.
In a free society, consenting adults should be able to settle disputes however they like; this might entail recourse to a coin toss, Sharia, Beth din or whatever. The job of UK law is merely to ensure that consent is free, informed and not too onerous.
This means that Sharia shouldn't and needn't be part of English law, but merely of civil society.
Similarly, the task of restraining cruel and thoughtless speech - insofar as this is necessary - should fall to civil society, not the law. Whatever the response to hatemongers like David Irving should be - protest, debate, ignoring him - it should not be the law.
Dr Williams, then, is failing to see that some things - many things - shouldn't be done by the law, but by free people.
But is the error his alone? I fear not. When the law is increasingly being used to "signal" what sort of behaviour the state deems acceptable, we are losing sight of its proper function is a free society - to allow people to freely choose their own lives.
And if people cannot "relate" to this function, or if it conflicts with their religious loyalties, then tough. Some things can't be negotiated.