You shouldn't buy a conservatory if you want the 'all year' living space of an extension- it is supposed to be what it says, ie a garden room which doesn't require heating.
Best placed on east facing walls or even north facing, they heat up quickly in the morning and stay heated from direct sun. Conservatories are not intended to be open to the house without intermediate doors...the incredible heat loss in winter make it a waste of money to heat + if they have permanent heating they are treated as house extensions as far as the rating, planning and building regs departments are concerned.
External louvred shading on the south roof slope can shade summer sun but allow the weaker winter sun at the lower azimuth through. External shading is better than internal blinds [which just retain the heat once inside] as they prevent the heat of the day getting in to overheat the space. Double glazed glass roofs are fine especially with Pilkington K glass fitted on the inner leaf but make sure it's laminated glass on the outer leaf otherwise a heavy snow fall off the roof would smash it... or fit snow boards at the house eaves.
Don't waste money on radiators to use the conservatory as a winter room. Think of the conservatory as a garden space to use when the sun has warmed it up. If t gets too hot open the vents and doors and let the heat escape.
