wow, the UK sure has very tough gun control laws. Under section 1 of the "Firearms Act, 1968", possession of a firearm and/or ammunition without a valid certificate is a criminal offence, either one of which is passible to emprisonment for up to 7 years. sentences ranging from 2 to 5 years are common:
http://www.cps.gov.uk/legal/s_to_u/s..._firearms_act/
Nightingale might have had an argument on the pistol. If he could raise a reasonable doubt that he did not know he had it, he might technically have been held not to have been in "possesion" in contravention to the act.
..but what about the ammo? Did'nt he wonder what was in the big plastic box under his bed?