as you have a tank and pipes in the loft, there is considerable risk of a freeze, burst and flood.
I presume your loft is well insulated. This means the loft will be at outdoor temperature (which can easily be -10 on a winters night).
There is more risk of freezing when you are away, because you will not be using taps and causing water to flow in the pipes, which can bring warmer water into them. Even if the pipes are well lagged (which they should be) it slows heat loss, but does not prevent it. It may take several uninterrupted days of frost to freeze a loft pipe. An unprotected pipe can burst in an hour.
There should be a gap in the loft insulation under the cold water tank, so that some heat can leak up to it, but the pipes will most likely be on top of the insulation. The tank is unlikely to freeze, but a pipe may, most often at a control valve or elbow that is exposed through the lagging.
You can't drain some of the pipes, such as radiator pipes under the floor.
So:
leave the house heating on. This will prevent freezing inside the house, and some heat will escape into the loft.
Turn off the stopcock in your garden or under the pavement (water meters usually have a screw-down handle beside them.
Turn on both hot and cold kitchen taps and leave on. They will run dry. You can then turn on the bathroom taps. Very little water should come out.
This will drain water out of the pipes, reducing the risk of a burst. It will also drain most of the water out of the loft tank. This means that if one of the pipes supplied from the loft bursts, there will only be a cupful of trapped water in the pipe to leak out, rather than twenty gallons from the tank.
Next:
Set the thermostat to 12C, no less (your insurers may possibly specify a different temperature for an unoccupied house in winter, look at your policy).
Set the timer to 24hours continuous for both CH and HW. You might think this expensive, but remember that both the cylinder and the CH have thermostats.
So the heating will actually only come on if the internal temperature drops below 12C. In most parts of England, some of the time this may only happen during cold nights. On sunny winter days solar gain through the windows will keep the temperature somewhat up.
The cylinder will only be heated when it drops below temperature. As you are not using the hot taps this will probably cost you less than 10p a day, as a little heat leaks out. It will warm the pipes around it, and a little will leak up through the ceiling to the pipes above.
On you return you will turn on the main stopcock, and then (when they start to run) your kitchen and bathroom taps. Tie a label round the kitchen tap "stopcock turned off" and write where it is, in case somebody else comes into the house on your behalf.
The heating system (possibly) has its own small tank in the loft. This will be slightly heated by movement from the water feeding the radiators and cylinder, so no need to drain. It does not hold much water anyway.
If you are unlucky enough to get a burst pipe after draining the tanks and turning off the supply, there will less damage then if you dropped a bucket of water, and it is easily repaired.
Otherwise, it can cost thousands.
