If the units are gone (misted up or condensing inside the 2 pieces of glass) in your upvc windows it is cheaper and easier to replace just these. Do your current frames have any weld failures in which case new windows may be your option. Any gaps between the frame and the brick are easily filled by a competent diyer, checking as well that the window is still properly fixed.
The area at and around a window will be chilly whatever you do. Be sure to invest in some decent thick curtains, and keep them drawn when its dark. You should notice an improvement in your comfort. This is much cheaper than new windows, which may reduce heat loss by a fraction more than your new ones, but is it really worth the outlay.
I used to sell upvc windows. As the elderly trusted a woman I often visited them and explained the cold area and thick curtain situation to them. It nearly got me the sack but I slept nights.