Three do allow tethering on their One plans (PAYG and contract). We're in a new build house and VM haven't finished finalising their cable lay. Everything's in place now (finally after 8 months) but they haven't added our address to their database yet. In the meantime we've been tethering to a Three PAYG sim via an Android handset and pull literally tens of GB a month (including downloads via NNTP). Some nights I pull 20GB at once overnight at 20Mbps!
From Three's blog:
I double checked with the manager of a local Three store and he said it was fine, fill your boots. Works for us.
EDIT: On Three you'd need to top up with £15 a month and use it to buy the £15 bundle which is the cheapest one which incorporates all you can eat data. Definitely do NOT go with GiffGaff their service is appalling. I was banned off the network for using 'too much' data at 2GB in a month non-tethered. Apparently my peak time usage was too high. Pathetic!