I probably wasn't clear. Its PAYG. But you can buy "bundles" (Goodybags/Gigabags) which last 1 calendar month.
Like all PAYG networks, the SIM has to be seen as active every 6 months (even if thats receiving a text, and turning the handset on to recieve it).
PAYG data charges are 20p for first 20Mb a day, then 20p per Mb beyond that. That makes it dear to use PAYG data.
Also, if its purely for data, O2 doesn't have great 3G coverage. O2 also have a fair amount of their 3G on 900Mhz, which older devices do not support. O2's coverage map is pretty accurate IME. O2 does have fantastic 2g coverage, if its for calls.
As said earlier, if using for data, and 99% of the time, you won't use data, but then if you're away in the pikey wagon for a week, expect to use data, leave it as PAYG most of the time, and then buy a Gigabag/Goodybag just before you go away (this will last 28/29/30/31 days). If that makes sense.