I am replacing my windows with double glazing lowE. Can report back on the workmanship in a month or so. Scouted for many companies, used the recommendations from the forum and got in touch with most of them.

Some gave poor consultation or simply have no idea on the glass specs they push on me, some dont even know the difference between U-value and UV-transmission, some cannot respond to inquiries at all or communicate with their staff. This was my experience with the larger providers for alluminium and uPVC windows. Some of the frames offered were very very convincing but if the company has very very limited knowledge about what sort of glass they put inside that frame this is of little use.

I also found companies with great glass but rather unsuitable frames with super old style locking mechanism.

I the end found an in between solution where I placed more importance on the glass than the frame as this was most important to me.

If installation etc. is good I will place a recommendation here.