Only a few large companies will manufacture their own double glazed units, with the rest being supplied by specialists who support the wider building and glazing industry. All my glass is sourced from such a company, which has been in the business for over 25 years. They produce my double glazed units to British Standard EN 1279 Part 2 & 3, just as they do for many other companies, with the same 5 year guarantee on their units
I specify an additional thermal coating on all of my glass by default. This helps to keep your home warmer and protects your furniture from the fading effects of UV light. This is generally equal to or greater than the glass it replaces. If you know that your glass has a higher specification, such as an advanced coating or is argon gas filled, then I can add this to the order and price for it accordingly