The cheapest way to get o2 data sim with NO contract tie in!
The device is £60, pay £60 upfront and get 40GB of data for £25 a month or 20GB for £17.50 a month with no contract period thanks to the delight of o2 Refresh! Or just pay £9.99 upfront and add £2.00 a month to you're bill knowing again with Refresh you can leave anytime ;)
You can get the double data offer on o2 mobile broadband products but these are NOT refresh plans, you HAVE to see out the full 24 months or buy the remainder of the contract out, with Refresh you are free to leave without penalty termination charges.
Obviously this wont suit everyone but if EE, Voda and 3 signal is restricted to you then this is the cheapest you will get o2 data sim as a alternative to landline broadband. Just place the sim in a usb modem or mifi unit or the dual 4G wingle (modem & wifi - can plug into a rechargeable battery or usb port) for £35 payg https://www.o2.co.uk/shop/mobile-broadband/huawei/4g-dongle-with-wifi/#contractType=payasyougo
I expect this will go cold by those lucky enough to not have just o2 signal out in the sticks ;)
Double data terms: http://www.o2.co.uk/termsandconditions/rewards/christmas-2016-double-data-selected-tablets-2gb-tariff-and-above-terms-and-conditions