The decision on whether or not to heat a glasshouse resides with the greenhouse owner. A totally unheated greenhouse is unlikely to offer more than minimal protection to overwintering plants and is inadequate to protect frost tender specimens.
A standard 8 x 6 aluminum greenhouse un-insulated and unheated will barely offer a degree or more of frost protection to any plants inside. If you have a few tender plants you would be better moving them into a utility room, a bathroom or somewhere light and frost free within the house.
You can provide plants with a more protected environment by adding a bubble wrap double-glazing to the internal panes of the glasshouse. This will help, but be sure to allow opening vents the space to open on sunny days to afford vital winter ventilation. On really cold days you can consider throwing extra fleece over the greenhouse before night fall but you need to remove this in the daylight to allow the winter light to penetrate for the benefit of your plants.
Wrap fleece, hessian and or bubble wrap around large pots of susceptible plants and stand them on polystyrene sheets or thick layers of bubble wrap for insulation from below. Be aware the slugs and bugs may also like the protection of the layers of insulation so use your preferred method of control. Always be ready to remove insulation during sunny or milder spells as it may trap and harbor some greenhouse diseases or problems.
Fully glazed greenhouses can be additionally insulated by paneling the bottom row of glass panes with timber sheets or similar, at floor level. This can be done inside and outside the greenhouse. Use a method that allows the panels to be installed and removed easily.
Large glasshouses can be divided into smaller areas by creating a greenhouse within a greenhouse. Do this by moving a cold frame into your greenhouse to house more delicate plants, or hang a curtain of bubble wrap across the whole greenhouse to create an inner-tented area for plants.
Make sure that your greenhouse doors fit properly, that all cracked and broken glass is repaired or replaced and that any draughts are blocked up appropriately. Keeping your plants protected is just as much about insulating your greenhouse as actually heating it. In fact you may find you can actually get away with only heating a small area, such as a greenhouse bench or a propagator to keep your most precious plants frost-free. Use a system with a thermostat to prevent things getting too hot or too cold, it will also save energy and ensure that the heating kicks in only when needed.
Think carefully about where you site any form of greenhouse heater and the type of heater you choose to use. For reliability a thermostatically controlled electrical heater is a great choice, but you will need a dedicated electrical supply and a splash proof greenhouse suitable socket. Baring a power cut this will cut in when the temperature drops to a particular level. Keep the heater away from anything that will block its heat or catch fire and check on the greenhouse and your plants regularly.