There are many things that modern sash windows now do that save energy over the traditional single glazed sash window.
Double glazing and argon gas creates a barrier against the cold by reducing the thermal conductivity as compared to normal air.
There is Special glass called Low 'E' glass this keeps in heat in much more than normal glass, in addition there is a warm edge spacer bar between the glass along the edges this helps the internal glass to stay warm and lower thermal transmittance.
draft excluding strips - reduce and practically eliminate any draft between sashes this along with hard wood frame and sashes correctly installed with double glazed units can have a dramtic cost saving on your energy bill.
There is an unforeseen benefit to double glazing which in central London can be very welcome and that is the double glazing reduce traffic and external noise, achieving a decibel rating of 42db.
If having your sash windows updated to include the draft proofing system and low E glass glazed units. then the next best thing would be secondary glazing or thick curtains that will keep heat in but not nearly as much as properly made and fitted sash windows.
