This is one of Burncoose's particular mail order specialities and we are very happy to make more specific plant suggestions with a few pictures of your own glass covered areas.
The key thing to remember, if your conservatory is heated with radiators and actually part of the house, is it will have very low humidity levels. This makes it unsuitable for those tender greenhouse plants which only thrive in moist humid conditions and not in hot dry environments. The plant choice in these conditions is more limited.
However, if you have a frost free greenhouse with some heat, good ventilation and plenty of moisture in the air, then you can grow a far wider range of plants only providing heat when the temperature threatens zero degrees, below 5 degrees if you are more cautious.
To give colour through each season requires quite a bit of thought about the plant selection based on ultimate height, the ability to prune and level of shading.