If it had ever acquired a roof, it would have been the largest church built in Britain between the Reformation and the Great Fire of London.
One of the lecture's themes was the feedback between architecture and (with apologies to OMD, not morality but) theology. The post-Reformation English church inherited a stock of Catholic church buildings, which were adapted more or less thoroughly to Protestant use but whose history could not be shaken off. When Protestants designed and built new churches for themselves - as they did of necessity in France, and as they sometimes did in Scotland, where the existing church buildings were often very inadequate - they came up with very different buildings, preaching houses which would not have tempted anybody to reintroduce a ceremonial life. Take the one in Burntisland in Fife, for example (Andrew spent a while on this one with us):
So did the English church remain relatively conservative and ceremonial in its Protestantism because it was adapting existing buildings; or did it adapt existing buildings because it was relatively conservative? Both, no doubt. But I was left with a counterfactual question:
Imagine that the Great Fire of London had happened in 1566, not 1666. And that half of the city's churches had needed rebuilding in the later 16th century. English Protestantism would have been forced to develop an architectural style of its own, which given London's importance, would have become a model for the rest of the country (as 'London style' did after the Great Fire). - How would that have changed English Christianity? I suspect, profoundly: the country would have been set on a more explicitly Protestant, even Puritan course culturally. And given the huge tug of inertia which architecture exerts, it would have been difficult to divert it thereafter.
Alternatively, given the financial starvation of the post-Reformation church, the penny-pinching penury of the Elizabethan regime, and the known fact that old St Paul's was left half-ruined for more than a century before the Great Fire, fifty historic parish churches might have been replaced with a handful of jury-rigged barns, of which we'd be ashamed down to the present.