Small churches make wonderful venues and provide a most intimate experience for worship. There is much more to it than just the acoustics though with regards to it not being done in the U.K.
As for house concerts... I have to agree that houses in Britain are much too small although 20 guests at a house concert is a reasonable figure.
I'd be quite happy with that and would expect to sell 20 albums.
Get into the community hall and you can expect to sell one album for every 10 patrons.
Coffee shops... again, not going to sell so much (around 10%) as a house concert but still decent.