I stand corrected, you can get a 3 phone with "unlimited" data, which does allow tethering for under £15 a month.
But when you get into the small print you find...
6.5 You must not use Three Services, the SIM or Three
phone number or allow anyone else to use Three
Services, the SIM or Three phone number for illegal or
improper uses. For example:
(d) to download, send or upload content of an
excessive size, quantity or frequency. We will contact
you if your use is excessive;
9.1 We may Suspend any or all of the Three Services you
use without notice if:
(b) we advise you that your excessive use of
Three Services (as may be defined in accordance
with Section 6.7 above) is causing problems for
other users, and you are continuing to use Three
In other words, you can use as much as you want unless we think it's too much and we don't have to tell you how much is too much up front.
Before you sign up though, make sure you have a decent 3 signal indoors.
And if someone takes the phone out you'll have no internet at home.