I'd just buy a cheap Nokia, charge the battery up, and put in any PAYG and topped-up SIM card, take out the battery, and hide it away. The battery should last a couple of months when disconnected.

 

Though planning like this seems a little extreme; though you're right-wing I never really thought of you as the Survivalist sort. If you're planning on leaving civilisation behind for a while you'd be better off with an Emergency Radio Beacon and Radio than a mobile phone.