Q: When and where might my wife and I attend a nice Christmas or Christmas Eve church service? The religious denomination is not as important as knowing that the service will be in a beautiful setting with excellent music
A: There are lots of churches around central London so your choice may depend on where you are staying. Some ideas include Southwark Cathedral, St Martins in the Fields (I think this is its name, its in trafalgar square), the small church at on piccadilly, just off piccadilly circus, even Westminster Abbey may have a service open to the public on those days.
When I was in London, Christmas 1980, I took in Midnight Mass at Brompton Oratory. Quite wonderful!