In answer to the above comments, you use the data the following month when you get a good data connection. It is frustrating not being able to use your allowance because of inadequate UK network coverage (any network).
if the data connection is bad the first month it may be the same the second month how would you know if any better in the second month. (any network)
I would like to chime in and say there are certain use cases where it would be very useful. Personally, I live in a place with shoddy wifi and cannot steam youtube in HD or stand any chance of gaming (beyond Worms Armageddon). So if I were able to rollover my unused data from previous months, I would be able to put it to use and enjoy a week of HD netflix for example.
Another point being, which I feel strongly about, is the fact I paid for the data in the first place.
Really in an ideal situation, any unused minutes/data/texts should be rolled over until I need more. But that just isn't going to happen really in this world