Why not buy a PAYG iPhone online, then return under DSR. They will refund you for the iPhone, but not the SIM. You keep the SIM, top up as normal for your calls/texts, but get 12 months unlimited data for free. The Carphoen Warehouse confirmed it was kosher for me just to use the SIM with the original data plan.
I'm doing this now... not because I planned it this way, but the iPhone I had was faulty and I sent it back for a refund.