Good grief. Disappear for a couple of days and a few people over from Chatterbank high on opinions but thin on practical experience turn up to start a fight.
There are many ways to have a conservatory built.
One is to involve a project-led organisation that does only (or mostly) conservatories. Most of these subcontract base construction, assemble the superstructure as a kit of parts and fit all glazing. Roof glazing is either cheaper polycarbonate or proper glazing. They then subcontract any finishing tasks like flooring, electrical and plumbing (if required). At the top end of the market these companies make their own conservatory frames and glazing, smaller companies order kits from big trade producers like Ultraframe. Ultraframe also have their own supply division. There are many suppliers in this camp that have been in existence many years and are successful. Buying this route is probably more expensive.
Another route is to employ a general builder. He is generally a tradesperson of one specialism with friends and associates whom he employs for other skills. He project manages the build. He will also specify a kit frame, based on your requirements, will not create the frame from scratch but buys it trade.
The two routes described are thus similar but the second one is likely to cost less as it involves no salespeople.
Seek references from satisfied customers for either of your chosen routes. Both are suitable and could end as a great success or an unmitigated disaster.
Installing double glazing units is not a specialist task, and to suggest it is demonstrates a lack of knowledge. Units are held in by rubber gaskets, with beading strips trimmed to size on the outside. Note the lack of detailed knowledge in the pontificating above, and compare against someone who has done it, hence knows how it works.
Finally, don't forget planning consent and building regulations. Conservatories may require consent but may be able to be built under permitted development rules. Your installer or general builder will be familiar. Building regs are not required unless the area is greater than thirty square metres, but you must not remove the original external patio or other doors. There are good reasons for this. Conservatories lack heat like a sieve and you will make the whole of your house cold.