Should I think about insulating a cheap conservatory? if so how to do it

What kind of construction is the floor? It seems to me that the floor would be your main heat loss culprit if you're up to a sufficient insulation value in the walls and ceiling.

To answer your question directly: There are no ceramic or composite tiles that significantly add to the R-value of the structure. However, there are additional materials that you can place inside the walls.

I would look into other things, like heat loss through a window, or through a fan vent that draws air, or through the floor. If the room still feels cold, I would definitely include in your renovation a heated floor mat, which gets embedded in self-leveling cement and then tiled over. Having a nice warm floor in the morning when you get out of the shower is one of the ultimate simple luxuries in my opinion.

The simple answer is "yes". Every piece of insulation you put in is a bit of heat that's not lost to the outside world. This is better for you as your house is warmer and your fuel bills are lower, it's also better for the environment as you're not using as much energy.

The more complex answer is "it depends". Buying and installing the insulation costs money (you don't say!) so the immediate equation becomes "is this cost less than the money I'll save over X years?". It's the answer to this that will, largely, affect your decision on whether to insulate or not.

The company who wouldn't estimate may be right (for their system) in that it would cost $2,000 (say) to do the work, but you'd only be saving $20 a year on your heating. This would mean it would take 100 years to pay back the cost. Now, if you can find a system that costs less then the proposition becomes more economical. Insulating the ceiling/roof space is a good thing to do anyway.

However, if your primary concern isn't the savings then you might decide to go ahead regardless.