I don't know much about glass coatings but I'm going for for a ceramic coat which requires the car to be decontaminated and machine polished first before the coat being baked on in a infrared oven, went with it due to the large amounts of positive feedback.
Ceramic coats are supposed to last 5+ years and offer scratch protection, uv protection and make your car a lot easier to clean as dirt and dust can't bake on the same.
It's a detailers In Doncaster here's there website
http://www.cleandetail.co.uk/gyeon-mohs-quartz-ceramic
Total cost is around £500 but bear in mind my VW dealer wanted £450 to have there shop put autoglym lifeshine on (a great product I've been told, just applied usually wrong due to lack of time/training/pride)