I like how churches are always so cool. =)The vast majority British buildings are built using brick instead of wood or stone and, on top of that, insulation material is heavily used to keep the heat in. In fact a few years ago there was a government scheme to install free insulation into people's attics and roofs which, whilst great in the Winter, does not help at all in the Summer.
Shouldn't the insolation also keep heat out? Do you have these shutters for your windows to keep the heat out during the day?