I pay around £40 month on three. Contract has also ended. Need to give them some grief I think!
I pay about £26 per month with 3 at the moment and get their 'all you can eat' data allowance which I need, as I gobbled up about 18GB last month.
When I went in to see about getting my phone fixed they really didn't want to know, so if I can get an unlimited internet deal elsewhere that's where I'm headed! 
18Gb is a fair whack, presumably thats 4G as well, how old is the contract, the old ones let you tether, any new ones now restrict tethering to 2GB, the 'All you can eat' is now only available for Phone use.
For that sort of Data requirement I would do your Homework before jumping in with another Network and New Contract, and a must is to review your Coverage as that can really kill off good or bad Data Throughput speeds.
You only really have 3 choices, when it come to Unlimited, Three, GiffGaff and T Mobile, forget the likes of International UK Sims offering Unlimited (the likes of Libra etc) as you be lucky to get more than old Fashioned 3g speeds (0.3Mbs).
Three, you already know about, GiffGaff offer Unlimited £18 but its capped at 3G /HSDPA only and your have tethering issues, T Mobile again offer Full Monty still (2000min, Unlimited Txt and Data) for £21, which if you shop around you can get for £15pm, again its 3G / HSDPA only (capped at 4Mbs, a bit more between 1am and 8am), good thing about T Mobile, is there not that strict on Tethering, Dongle, WiFi Hotspot use, even though they say you cant.
Other option at the moment (untill end of March 2015) is EE, which are currently offering Double Speed and Double Data on some contracts, ie take out a plan with 10GB and get another 10GB free for the contract etc, plus they give good 4G speeds, and Tethering is allowed (at the moment).
Another Possibility, is if your a Virgin Media Customer (TV, Broadband etc) they offer Unlimited Mobile Data Tarriff (again Tethering is not allowed). Or if you upgrade to EE Home Broadband you get a free upgrade to your EE Mobile Plans, either +10GB or +20GB cant remember which now.