A simple plastic or aluminium vent & ducting hose would cost only a few pounds. A vent slate for the roof - maybe £20-25. Then you have to get up on the roof.
Allow £100 if roof access is easy. Could be more if it gets fiddly. You'd need either a roofer or any general builder.
The trouble with houses built in that period is that they are so cold. Probably solid brick construction. It's a well-known problem. It was Ok when we lived with a good draught, but today, cold walls attract condensation.
I dealt with one recently by fixing battens to the outside wall, board insulation between the battens, then plasterboard & skim. Re-fix the skirting board and you're done. Not a big job at all, but it raises the comfort level in the room considerably.