Our house is listed. We cannot replace our single glazed windows.
What we have done, on windows that are non-openable or treated as non-openable is get a piece of glass the size of the surrounding window frame and fitted a flexible plastic strip around it, with clips that are screwed onto the window frame. www.tubeway.co.uk/easyfix-diy/easyfix-glazing.html
On windows that are usually only opened in summer we have had rigid acrylic sheet cut, as above and we have then stuck special magnetic tape www.amazon.co.uk/Adhesive-Magnetic-Steel-Secondary-Glazing/dp/B00CI5T8PS onto the window frame so that we can put the interior double glazing pane on the window in the winter and peel it off and store it in the summer.
Neither system is obtrusive when installed - unless your window frames are not painted white!
NHS U turn on trans terminology