A rated upvc windows

[packadarm]

How do I identify A rated windows offered by various window installers

[buildersmate]

Everything you need to know about it is here.
http://www.bfrc.org/consumer/index.aspx
... except one thing. This is a voluntary scheme run by the BFRC and the BFRC is a subsidiary of the Glass and Glazing Federation - the trade body for windows manufacturers and suppliers.
So I suspect (but I'm not sure) that only members of the GGF will be using the scheme and not all glaziers are members.
An A-rated window system will have sticker-based literature similar to the codes that have sprung up on fridges and other domestic appliances in the shops - with the codes A to E.
An A-rated window will definitely need to be using low-emissivity glass in its construction - that is glass coated each side with a chemical that improves the thermal resistance. The frames will alo have to be constructed to excellent thermal standards.