Obviously St Paul's comes to mind but are there better, possibly less crowded options in Central London? I saw mention of St Martin-in-the-Fields in another thread for their concerts, which would be closer to where we're staying (Westminster) than St Paul's.
We'd be looking primarily for aesthetics and a good location so we're not taking too much out of our day traveling to and from mass, but avoiding crowds (likely impossible on Christmas Day) would be a bonus.