It appears that there may be one unmetered 4G option in the UK and that is via three who plan to launch their 4G service in December 2013 by allowing customers with 4G capable handsets to keep the same tariff and gain the option to use 4G where it is available.
This means that people on The One Plan which can costs £37/month with a Samsung Galaxy S4 will give you an all you can eat data allowance. Research suggests there is no fair use policy, or put it another way we could find no trace of one applying to the product, and other parts of the three site say no fair use policy. Of course if the firm finds that too many TeraByte per month (1000 GB) customers join their service this policy might change.
The question now is whether this unmetered stance from three will be enough to force the hand of the other operators, past experience in the 3G market suggests probably not.
Update 3:30pm The details of the announcement by three are over on their own blog.