"We're all told to wrap up, insulate and generally stop the heat from escaping else we'll turn up the gas and kill more polar bears. Tell what I'm supposed to do then for Summer? Strip out the cavity insulation, knock a bloody great hole in the wall, get bigger windows that open wider, install an air-con unit which will use shed loads of leccy and kill more penguins and polar bears?"
No, pile on more insulation so you can use a smaller air conditioner and conserve electricity. Insulation works both ways - it doesn't care if it's stopping heat from escaping or blocking joules from slipping over the border into the house.
The typical housing design advice in the US South is as follows:
1) Moar insulation
2) Double-paned (double glazed?) windows. If you're feeling like an early adopter, try low emissivity, UV-blocking windows.
3) No, you don't want bigger windows and doors (though large windows and single-paned sliding glass doors are common in the US South.) Of course, Britain's summer air is usually about the same temperature that the US South hopes to achieve through air conditioning, so you'd want that stuff flowing through big holes in your walls. Maybe you should ignore this point.
4) Control drafts - spray-foam insulation is preferable to fiberglass batting because it prevents air movement in the walls and into the attic. (Spray foam insulation is also more flammable, so you have a ready supply of fuel for your fireplace the next winter.) Check your doors and see if they have any significant gaps that can be sealed with a strip of insulation.
A well-insulated house is a lot cheaper to air condition than a poorly insulated house. My summer electrical bills dropped in half moving from a home made in the 1970s to one made in the 1990s.