My colleague Symon Hill's appearance on 4though.tv this evening (13 April 2011), arguing that the mutual inherence of an Established Church and the institution of monarchy compromises the Gospel message of freedom and identification with the least in society (http://www.ekklesia.co.uk/node/14559 ), comes weeks away from the wedding of Prince William and Kate Middleton.
It was another Royal Wedding - between Prince Charles and Camilla Parker-Bowles (rendered somewhat controversial in ecclesiastical circles because the future Head of the Church of England and Defender of the Faith was a divorcee marrying a divorcee in a context of mutual adultery) - that gave rise to an earlier Ekklesia comment on this vexed topic.
With my co-directorial colleague, I penned a short article for the Church of England Newspaper (18 February 2005), on the case for disestablishment. Our case was, and is, thoroughly theological in character. It is not a matter of criticising individuals, or even of secular 'fairness', but of what it means to tie into the institution of monarchy a community of people who are supposed to be defined by their allegiance to Jesus Christ - a very different kind of king (http://www.ekklesia.co.uk/node/13613 ); one who subverts the whole notion of earthly 'kingship', as Professor Chris Rowland (http://www.ekklesia.co.uk/node/8020 ) and the Rev Tom Hurcombe (http://www.ekklesia.co.uk/node/8138 ) point out.
Anyway, this is an excerpt of what we wrote in 2005. It still feels timely today:
The Church’s own matrimony of convenience [to the Crown is] fine if you think the Church is there mainly to give the state comfort and a religious gloss. But the true cost is a loss of integrity in witness and prophetic engagement, turning pastoral care into collusion.
Defenders of Establishment say that the Church has greater ‘influence in the nation’ through its Crown privilege. What they really mean is that, in all honesty, we have much less trust in the Prince of Peace than we do in worldly princes.
As the Gospel reminds us, the state’s power is secured by its ability to crucify (or, these days, go to war). God’s power, however, is seen in Jesus on the Cross. You can see why the state looks a much safer bet. But it’s not where resurrection faith lies.
The message of the Gospel is that God’s grace and forgiveness is available to all, whatever their status. Jesus practised open table fellowship, said that the last would be first in God’s kingdom, and created a new community of equals – the church.
The Crown, by contrast, is an institution that exists to preserve an order based on eugenic privilege. That most Christians do not notice this, and do not see how the Church’s royal allegiance falsifies the Gospel, illustrates just how blinded we are by worldly status.
Establishment diminishes the Church of England’s ability to proclaim and live the Gospel free of state sponsorship. It inhibits equal relations with other churches. And it habituates Christians to trust in earthly power rather than God’s disarming strength displayed in Jesus.
[I]t was a famous controversy over divorce and remarriage 500 years ago that first institutionalised the arrangements for the Church’s governor to be chosen through royal heredity. Now there is a chance to think again about the wisdom of this settlement.
The Church can choose to remain attached to the monarchy and thereby assure its place in the heritage industry. Or it can embrace the opportunity to spearhead a bold ecumenical reconsideration of church-state relations.
The full article can be read here: http://www.simonbarrow.net/article81 
(c) Simon Barrow is co-director of Ekklesia.