We are thinking travelling to London a week before the wedding and wonder if that is a bad idea as far as crowds, availability, etc.?
i don't think it will impact your travel. The day of the wedding there will be parts of London that will be VERY crowded.
You may have issues travelling that week, not because of the Royal Wedding but because it's the Easter weekend, which will also cause crowding and engineering works on rail.
As Philip said that Easter is the weekend before the wedding, so Good Friday, April 22nd is a bank holiday and Monday April 25th is a bank holiday. But if you are there before the weekend you shouldn't have any problems with crowds.