Even though the building regulations have improved so much over the last twenty years, so that all conservatories now have to constructed using double-glazing, there are still huge temperature differences where it is too hot in the summer and too cold in the winter.
If you insist on having a conservatory the only way you can prevent enormous amounts of heat gain in the summer is so provide solar shading by using external blinds or louvers on the outside of the glass or blinds that are integrated between the two sheets of glass in a double glazed unit to stop direct heat coming into the space.
Internal blinds hardly do anything as the heat is already in the space.
Good solar shading will also have to be combined with good internal ventilation.
To prevent it being too cold in the winter make sure the conservatory is built using high performance insulation in the floor and low level walls, combined with high performance double glazing and a good heating system.
Personally if it were me, I would forget about a conservatory all together and build a beautiful house extension with lots of glass in the walls, but a solid tiled and insulated roof with large opening skylights within it.
The extension can be designed to be more in keeping with the architecture of your house and all of the problems of heat loss and heat gain in a conservatory will disappear.