Hi,
It depends on how long you are away from the boat.
The only really safe way is to drain the engine down, not forgetting the raw water pump. When I had a R/water cooled BMC I made a drain tap with a polypropelene pipe leading to the bilge so thet it could be drained easily. This just left the heat exchanger and raw water pump to protect, again by draining.
To avoid draining I also used to hang a small paraffin greenhouse heater by the engine, this used to run on a low setting for a week and protected the engine and R/water pump.
I converted the engine to a heat exchanger cooling system and this meant anti freeze could be added to the block, which made frost protection much easier.
A useful way of providing frost protection is to cover the engine with a blanket.
If your r/water cooling system incorporates a gearbox oil cooler don't forget to drain that as well.
It is also wise to block up ventilation holes in the engine bay.
I am not sure how many degrees of frost protection these actions will cover, but should the cold weather last for a long while, complete draining would be the only option.
It may well be that different protection levels are needed for different parts of the country, the weather being colder in the North (my boat was based in the south where temperatures are slightly higher).
If a boat should sink due to a pipe damaged by frost, it is doubtful if any Insurance company would pay out. You should have a sea cock as close to the hull as possible and this should be turned off when leaving the boat, especially in frosty weather.
A very thorough Surveyor gave me this advice when carrying out a BSS examination on the boat, although I moved mine it was not a requirement of the BSS scheme.
Also make sure that the engine is properly drained down if the boat is on brokerage in the winter months and make sure that the broker and any prospective purchaser are aware of this and should only start the engine under careful supervision and if started make sure it is drained down again - otherwise you (the Vendor) will pick up the repair bills.
As an aside, I was reading an interesting case where an insurance company would not admit a claim on a house badly damaged by a gas explosion because the homeowner had repairs on the gas installation carried out by a non Corgi registered person, one wonders when this attitude will be extended to boats.
Leo