ZD, I asked Dayan Ehrentreu Shlit”a at the time about Chief Rabbi Sacks attending William and Kate’s wedding. Dayan Ehrentreu is the Emeritus (former) Rosh of London Beis Din and thereby head of the Chief Rabbi’s Beis Din.
His first reaction was, “Er hot mir gornisht gefrekt!” (he didn’t ask me). When I asked him what the heter was, he said (Don’t quote me, because it was 6 years ago and I may have forgotten details in the meantime) firstly, Rabbi Sacks wasn’t seated in the main area of the church, but in an area that is added for large ceremonies such as weddings, coronations etc. Secondly, it wasn’t a prayer service as such but a wedding, even though prayer was offered.
Another answer may be that an Anglican church isn’t the same as a Catholic church would be, as has been mentioned many times on this forum.