Understanding consumer law pretty well....
If it is found that the cars produce considerably more NOX emissions here in the UK than the imposed limits, you could easily argue the product was mis-described as it was advertised as being green, also since it would be in breach of the regulations for NOX emissions, it therefore would not be fit for purpose along with being mis-described again by the implied emission levels supposedly being under the regulation limit. A simple argument that you bought the vehicle for its green credentials and the fact it should never produce more than X amount of NOX due to it having to comply to the regulations, would hold a lot weight.
In addition to the above, if they bring your vehicle in for a software update - it would almost be certain this update would affect MPG and/or performance - and the vehicle again would no longer be as described and not performing to what it was advertised to have been.
However returning the vehicle for a full refund would prove difficult if the car was over a year old - the argument of what was a reasonable amount of time would come into effect, and your use during this time would be considered.
What you would be entitled to though is damages in any depreciation of your vehicle due to it now not performing (emissions wise) as well as it should have. VW may be forced to pay all customers an amount reflecting depreciation losses in any class action law suit that will probably follow this scandal (should it be proven they have cheated the UK emissions tests)