You can also use mobile phone credit via premium-rate SMS message, but you only get 1 AW credit per £2 spent. Still worth considering if you just want to buy a few AW credits with your petrol, or if you have one of those PAYG deals where you have to spend £10 per month to get your Internet allowance and your phone credit keeps going up because you're not spending £10 on calls - in that situation, it's free money, as you'd otherwise have no use for the credit.
But you need to spend at least £30 in one go to get a freeview, and you can't do that with SMS.