In the recipe book we recommend keeping your wine between 22ºC and 27ºC, which is the perfect temperature for the yeast to get to work. Normal room temperature in the UK varies throughout the year and is usually somewhere between 18ºC and 25ºC.
Don't worry too much about the exact temperature of your wine, as long as it's kept away from direct sunlight, has a steady temperature and isn't anywhere excessively cold (e.g. a garage).
If you're worried about this then stick on brewing thermometers can be picked up very cheaply, they just attach on to the side of the demijohn and give you a rough idea of the temperature of the vessel.