I guess Suff is right about Ts & Cs. I pay monthly and get XGB of data. If I do not use it I lose it, it does not roll-over. On the 1st of each month it all starts again.
For a miniscule data user like me it is no big deal. If I was a phone drone glued to the screen and on apps 24/7 I might think differently. For me it is cheaper to get the unlimited calls and texts with a bit of data bundle for a few quid and lose the data than pay a monthly fortune for all the bells and whistles, or 4p a text and so much per min for a call on PAYG.
We all need to play the game our way. They still win.