If you're going to change to glass, then you will experience a fair hike in cost. there's a lot of shapes in a conservatory roof and all the glass has to be cut manually, unlike square/rectangle units which are done on a machine.
The roof bars also need to be of a heavier duty to take the extra load from the glass, so you're looking at a complete new roof. If you do go for glass, stress to the company you buy from that you want the sealed units made from Blue 'Pilkington Activ" (yes, the 'e' is missing on purpose :-P ) on the outside and 'K' glass inside with argon gas filling. This is pretty much the best way to make a roof unit in terms of anti glare (although if you get a lot of sunshine into it, you may need some sort of blind) and u value (energy effeciency), plus it has a self clean coating on it, once it's weathered in it does a pretty good job of keeping itself clean.
I'm in the industry and i would only use 'Ultraframe' roofs, so my best advice is to contact them either via their website or by phone (they're in Clitheroe) and ask them for a list of approved installers in your area.
I'd be happy to take a look myself if I wasn't about 230 miles from North Wales!!!
Also be happy to help with any answers to questions thrown up during the process.
If it helps to gauge price, I did a 'P' shaped roof about a year ago in glass and that was £4600 ish and thats a pretty detailed roof and was about 5mts wide.
Good luck