If the registered owner is lucky, it's an £80 fixed penalty (which is reduced to £40 if paid within 28days). However if the matter goes to court the maximum fine is £1000 (plus costs, plus a court fee of £150 plus a 'victim surcharge' of 10% of the fine).
As Eddie has indicated, fixed penalties or fines are in addition to the £250 release fee that would be charged if the vehicle was clamped. (The registered owner can also be ordered to pay the tax that was due for the untaxed period).