you will hear many companys say A* rating bla bla bla. just get a good quality system Veka, rehau etc and use a company that gets good quality glass units and you will have no problems for 10 + years.
Its when people get cheap systems and companys that cut corners that the systems are draughty and rubbers fail etc.
same with anything you get what you pay for i have been fitting the stuff for 6 years in that time not a great deal has changed…