You wont get a PAYG where it sits there till you use the data up unfortunately. They have all cottoned on to that now and want to make more money out of us. Voda used to do that where you topped up £15 for 1GB and it lasted till it was used up. If you were lucky enough to get one and still have it (like me) then it still applies.
Yus, really wish I had one of those. But as you say, they want to squeeze us for every penny so, I'm s.o.o.l.