In the UK even in the hottest summers, cooling isn’t really too much of an issue, although it may become more so in modern houses with high insulation levels, lack of thermal mass, and inadequate ventilation.
However, when it is hot it is worth blocking out as much of the sun as you can from your house, so as to keep the internal atmosphere comfortable without the need for air-conditioning units.
Having solar blinds, shutters or shading devices installed will of course do this, or you may like to consider growing a vine or other deciduous vegetation above or in front of south-facing windows. This not only blocks out the sun but has a cooling effect in itself – and looks good!
In contrast, during the heating months it is important to try and allow as much direct sunshine into your house as possible, thus replacing your energy use with passive heat!