if you have a modern house, it may be that you have a combi boiler with no tanks or pipes in the loft.
If so, you need not worry about freezing up there.
However, if you do have tanks or pipes (including pipes going to a bathroom which have been run in the loft instead of under floors), and especially if somebody has shoved a boiler up there 
then it needs to be kept from freezing.
Houses these days usually have a lot of loft insulation, which keeps the house warm but the loft cold. If the insulation has been flopped over the top of the pipes, this will help keep them warmer with heat escaping from below, but there are joins and gaps in insulation, so it is worth looking carefully, adding pipe insulation such as Climaflex, putting tank jackets on, and looking carefully at all gaps and bends where insulation tends to pull away or get disturbed.
The other common unheated space is under the ground floor. If you have pipes under there (very common) and if it was done years ago, or recently by a poor-quality fitter, these pipes may not have been properly insulated. If they are close to an airbrick, they can freeze in a single night. Same with any pipes in unheated spaces for example a shed or garage. Some people put boilers in garages, which are usually draughty and uninsulated. In this case, the pipes need to be very well lagged with the BS grade of Climaflex (which is as thick as your arm) and the boiler needs to tick along at a very low setting.
More modern boilers have an anti-frost setting which turns them on if they get too cold, and they run long enough to warm themselves. You can also get pipe-protection anti-frost thermostats and heating devices.
Insulation slows heat loss, but does not prevent it, and does not add heat. So pipes that have no heat will eventually freeze, however well insulated they are. Insurers tend to say that if you can't completely drain all pipes and tanks (which is very difficult, especially for radiator pipes) then you have to keep the house at 12C day and night, or they won't pay for damage caused by a burst. They are particularly cautious about houses that are unoccupied, for example if you have gone on holiday or it is a second home.
You can buy a special antifreeze for radiator systems which will protect the rads, pipes and boilers. It is not very expensive, but of course needs to be topped up if you have a leak or drain the system.
The worst pipe to have a burst in, is the incoming water supply that goes into a loft tank, as the main will provide an unending supply of cold water at high pressure until it is turned off, and it will cascade through the house.