ignore the A, B, C etc rating its a load of old baloney
instead ask them for the actual U value of the windows they are supplying - this is the thermal transmittance value (or heat loss) - the lower the number the better
The maximum at the moment is 1.6 - if they're higher than that ie 1.7, 1.8 etc then they are not conforming to the latest minimum standards
you will pay big bucks for windows around 1.1-1.2, but anything between 1.3-1.5 will be good
also check that the fitter is "FENSA" registered - its a scheme that syas they are "trained" and allowed to install without going through building regs
if they are not FENSA registered, then they must make an application to the Local Authority - there's usually a minimum charge of anywhere between £50 and £150 (depending on the LA
hope that helps