We paid £3k for roof blinds from Thomas Saunderson last year. It has reduced the heat and greatly reduced the glare. It still gets hot, don't get me wrong, but at least it is bearable most of the time now!
The conservatory is about 4x4mtrs with Edwardian style roof. We have roof vents but only top openings on windows.
It has made a remarkable difference, but I don't know whether it has made £3k's worth of difference!
There are cheaper blinds, but to be honest we didn't think it was worth going cheaper. The quality of the material wasn't anything like what we now have and we needed it to reduce the heat, not to just cover the roof IYSWIM. Our original quote was about £12k!!
But at the end of the day we had just paid out all that money for a conservatory we couldn't use for a couple of months each year so £3k seemed almost justified.
We have also found that the heat in the winter is retained for longer. The room heats up OK with a rad and an additional small heater, but we found it very quickly got cold again because all the heat went straight out the roof! Again , it isn't brilliant, but vastly improved by the blinds.
The underfloor heating does prevent the heat going out the roof. If your feet are warm the rest of you is too.
But for the two rooms I would need 42ft by 12 ft's worth of blind - I bet that would be expensive.
In the smaller room I'm thinking of doing a DIY job with banners of linen (type) material on velcro to reduce glare