By far the best way to choose your new or replacement windows is to visit a showroom where you can see clearly the product and to be able to ask any questions about the products performance and finish.
All fabricators and installers of aluminium products buy their aluminium profiles from a 'systems' company. These are companies who design and rigorously check their designs meet with all the UK and EU legislations.
As a rule of thumb, choose a supplier who sources systems from a CAB member, a list of systems members can be found on the CAB website at:
If you have neighbours or friends who have had good experiences in having aluminium windows and doors fitted it is worthwhile going to their supplier to see if they can supply the product of your choice.
Links to further reading