Either pay monthly or PAYG, generally you need to buy the dongle on PAYG (about £30). You could go via quidco.com and get cashback on O2, either £25 back for PAYG package or £60 back for monthly contract. O2 currently have half price offer on dongle, £14+, also be sure to check coverage - http://www.ofcom.org.uk/advice/guides/bbcoverage/

http://www.ofcom.org.uk/advice/guides/mobilebb.pdf