If your house has a ridge vent then just leave the loft hatch open during hot spells. The chimney effect will allow hot air in the loft to escape though the ridge and draw up warm (but cooler) air from the upstairs. Works fine for me. If your house is modern and the roof is lined with a 'goretex' membrane instead of 'bitumen felt' then it won't have a ridge vent, so this probably won't work.
Plant a plum tree with a medium rootstock on any south facing wall and train it up the wall. This will provide free fruit in summer and the leaves will stop the suns rays hitting the wall. In winter the leaves fall off thereby allowing solar gain on a sunny winter day. Be careful not to plant it too near any drains though.
I had this back in the 80's. Bought it from someone called solar-X northern in Manchester and applied it myself. The downside is that on a gloomy winters day it cuts out a lot of light.