How good is double glazing.

There are many companies offering double glasing windows and conservatories, etc.
Is there an accepted standard which provides a quantative guideline of results, based on sound insulation and heat insulation of correctly installed double glazing?

No problem jumping in - that is what posts are for, I reckon, providing it is vaguely on the the subject.
Look up "secondary glazing". There are many companies, and a lot of DIY possibilities, to installing what is basically a second layer of glass inside the existing one. In some situations there is little alternative, as replacing sash windows may require planning permission, which may not be given.

A problem with sash windows is often also leaks around the frames and between them. There are many sofisticated methods of 'leak detecting', however wetting the back of your hand and moving it around the frame on a cold windy day is pretty good - and a lot cheaper. Then the DIY sealing can start - which is simple and can be very effective.