if you have a pound shop they sell some cheap stuff in there. £1. if you not running engine alot should be fine.
Cheap antifreeze may well protect from freezing to some level .... but I'll bet my bottom dollar that it contains few, if any, anti corrosion inhibitors, lubricity agents, or any other additives.
There is a REASON for using proper stuff, and it's not always just to make the manufaturers money, it is OFTEN down to compatibility with the systems/components in the car.
Read all the threads about oil cooler failures .. work out how often you want to change one, and how much it will cost you each time ... then add that to your cheap antifreeze price, because oil cooler failures are 99% caused by poor coolant maintenance ... ie .. lack of anti corrosion additives ... so the things rust away and leak.
It is your choice at the end of the day .. but advice like the above could, and will, cost you dearly in the long run.
Any advice should have an explanation attached, hopefully I have fulfilled this requirement.