I got an old Huawei E220 for free (they start at around £3 inc P & P on E-Bay), went on the net and unlocked it for free (a software task, but can also be done for £3 (E-Bay again) direct to the dongle) so it now takes any network sim (as opposed to the sealed USB units), I also took the 3.6 Mb speed limit off and reverted it back to 7.2 Mb but I've yet to have the opportunity to test that aspect as I,ve yet to get near the the 3.6 Mb speed let alone pass it!
Using the £5 "unlimited use" one shot monthly payment with 3, they think I,m surfing using a smartphone when I,m actually using a laptop so sometimes I have to top-up again when my usage is deemed "unfair" but you do get the free text/calls mobile phone stuff included.
If you want to play the 3 game "properly", it starts at £10 for 1Gb as previously mentioned but as this dongle is now not tied to any one provider you can shop for price/features/data/coverage and, if needed, you can swop with your phones sim (and vice versa)!
See, I knew that 22 years in the Royal Signals would come in useful one day!