I use my phone as a modem on Vodafone and you get 3 options, £1 a day - 500mb, £5 a month to the end of your contract - 500mb per day, or £7.50 per month and can cancel at any time, just pay for the month you need it - "unlimited" - note the ""'s before anyone says nothing is unlimited
OR you can get the dongle and pay £15 a month? I think it was last time I looked