Hi Duncan, and welcome to our forums
Windows 10 is a very robust operating system, but like all operating systems relies on being kept up to date.
If you try and prevent it from updating itself then you risk destroying the very robustness that keeps you (and others with whom you interact) free from exploits, and malicious code.
Its understandable that you wish to minimise the amount of cost involved in using your data stick, but it should not be at the expense of leaving your operating system "unpatched" and vulnerable to infection, exploits and other undesirable things that the bad people dream up.
Windows updates are really essential if you wish to use your computer on the Internet.....
If you really do not wish to have Windows updates then the most effective and safest way is to immediately disconnect the computer from the Internet and never connect to it again...ever ( not even just for an hour or two.) You should also completely isolate your computer from any external source of data, ie CD's Dvd''s, memory sticks, any USB or other type of gadget plugged in to your computer etc.
Obviously this complete isolation does remove a lot of the functionality that most folks want to use in their computer..... but it will also remove the need for you to top up or use your data card!!