The frame sections although hollow are divided into chambers, this helps with insulation as the inner surface is not indirect contact with the external temperature.
Beading to hold the glass is better for security if on the inside. (outer beading can be removed along with glass)
Depending on which direction you property faces choose the glass to suit. Glass can reflect external heat rays or allow them through. If your house faces a major road with a lot of noise / vibration, thicker glass in the units can reduce this.
Ironmongery, are they quality multi point locking (look at reviews oftype /make)
If you have panels below windows (or in doors) what is the thermal rating of the panels = efficiency ?
Replacement windows are made apprx 10 mm smaller all around to allow for fitting and plumbing / leveling, do they
foam fill the space between frame and masonry ? many don’t like to do this (extra work that needs to be done carefully) This is in addition to mastic pointing to the face and scribing pieces if walls are badly out.
With regards to FENSA I had a major problem while the windows where being fitted,( a total ba*** up )when I contacted them they did not want to know, they would not come and see the work unless I paid a considerable fee.
I had to sort it myself, get other estimates and arrange for the new company to remove the (crap) windows and doors and I had to recover my money.
If you are offered internal window boards/ replacements, check to see if these are just thin UPVC covers over the existing window boards,they are about 8 mm thick and very soft and damage easily.