Well I was wrong, New Kind of GP got this easily. It is a Palm Pistol, a .38 calibre single shot firearm, designed for those who have insufficient manual dexterity to handle a conventional handgun. A triumph of occupational therapy. As NKGP points out it is available on prescription in the US, and the cost can be claimed against medical insurance. It would without doubt be illegal to possess one in the UK.
I owe NKGP one beer.