Yes that is correct, as long as the phone is used I.e. 1 outgoing, or 4 incomming calls of more than 10 sec, or a top up, within the 6 months period then the sim will not be disconected.
I would add though, that it may well be worth getting at least the £5 (500mb) gigabag, for use with a smartphone, as data costs could easilly outstrip that on payg, with little use. (44mb in a day on payg would cost a fiver, thats not a lot of surfing, 20 web pages ish)
hope this helps