It does not say in the small print if you can or not. I doubt you can, because if you cancel the contract, most likely put you back to pay as you go. Any Vodafone users can comment?

No, once you cancel, you cancel. It's a rolling contract, not pay as you go.

Find it hard to believe that is a serious question (makes my dad look tech savvy), but search for "24GB three sim" - I think they were less than £30 recently - preloaded pay & go data sims that last the shorter of 24GB or 24 months. EE also do then, but much more expensive (think I saw a deal for 6GB/3 months at £9 - that had an RRP of £30 & street price £15 when I last looked). Note that EE will let you use them in phones, where shockingly three won't (i.e. use an old phone as a MIFI & three will suspend data)!

Yes it cost £9 from vodaphone to unlock.