Why would you choose to insulate your loft, and replace your windows with double glazing?
To save the environment and reduce your electricity bills it is essential to adopt methods that reduce the consumption of electricity.
Heating homes in winter uses a large amount of electricity. One of the reasons behind this is that only a small percentage of the heat generated by the heaters actually raises the temperature of the house. The rest escapes from the house to the outer atmosphere.
Heat rises upwards and escapes from the tiles that make up the roof very easily. One way to deal with this is to insulate the loft. In addition, normal windows allow infra-red radiation to escape easily. Double glazed windows on the other hand are made up of two glass panes with a small amount of gas sealed between them. These are very efficient in stopping the passage of heat through them.
By insulating the loft up to 15-20% of energy usage can be decreased. Using double glazed glass for windows reduces energy usage by 20-30%.
A quick note: when insulating your home, it is important to ensure proper airflow inside and out; it seems counterintuitive to let air in during the winter, but it's essential to keep the house from developing mold from Condensation. Here in Maine, my father's house is Superinsulated, and if we keep all the doors and windows shut all the time while running the heater, moisture develops on the walls and windows, running down to the floor and soaking in. We've had problems with mold and mildew for the last ten years or so, but we've been able to prevent most new damage by allowing a certain amount of airflow during the day and keeping the heat at a reasonable level.
Interior condensation can cause severe damage that may not be fixable without removing joists and beams. It is always better to consult with a building professional who understands your region's climate.