I don’t know much about them, but we had our windows replaced last year.
I think that most double glazing will be A or perhaps A+ rated. I am not sure if sash windows will make a difference, as the frame will have a bearing on the overall U value as well as the glass itself.
Everyone I spoke to reckoned that it in this country it is not worth the extra expense to go to triple glazing. There is not enough of a temperature difference between the inside and outside to warrant it.
Definitely have a look at what different suppliers offer; some are definitely better quality than others.
Make sure you get a few quotes. There was a massive range in final price for our windows.
And I would steer clear of places like Anglian and Safestyle. A friend of mine had Anglian and they seemed very expensive, plus there were loads of problems with the window frames, locks and hinges that took them months and months to sort out.
Safestyle quoted another friend of mine and it was more than twice the most expensive quote that we had for our windows, for less windows and doors. They then kept ringing back for weeks offering discounts and reductions.