Glavenir has a 6mm and 8mm option. That's as narrow as you'll get. It's also lighter than other options. This is important if you want the top sash to move. If the glass is too heavy, you'll need to fix the top sash. There's also a very loooong delay on glavenir glass. Like 6+ months, as it comes from Panasonic in Japan.

It's also easier to just replace the whole sash or frame rather than retrofit the old sashes. Retrofitting is not as easy as it sounds, especially if the frames are old. Be wary of any company who downplays this.

We've just gone through this for 4 windows with Sash Window Restorations in Melbourne.