You can get a Wireless Dongle on a Pay as you Go basis, just like a phone.
Check the normal mobile networks for coverage and you can get a dongle with an initial data "included in the cost" . Note that to get a sensible amount of data is not cheap.
You could use a 3 phone on one of their "all you can eat" internet plans - they allow truly unlimited amounts of data although, technically, only the most expensive one allows tethering but if you have a phone with an access point built in (lots of Android ones do) then I dont see how they would know!
Ex <n>ildram , been to SKY MAX - 15,225 Download
BE Unlimited - 21,000 Download 1,200 Upload ON THE LINE THAT SKY COULD ONLY PROVIDE 15,255 DOWN AND 800 UP ON!!!,
Moved house, now BE Unlimited 6,500 Down, 1Mb/s up - gutted!
FTTC Cab installation commenced 12th April - expect full 80 / 20 - bye bye BE, hello BT Infinity soon!