The law requires that you report the accident to a police station or 'to a constable'. Hopefully your phone call will meet that requirement.
Section 170 (2) of the Road Traffic Act 1988 states: "The driver of the mechanically propelled vehicle must stop and, if required to do so by any person having reasonable grounds for so requiring, give his name and address and also the name and address of the owner and the identification marks of the vehicle".
However, unless the shop was of the type where the owner lives on the premises, there wasn't anyone for you to give your details to anyway. That, and your fear of the occupant(s) of the other car, would probably be deemed by a court as providing sufficient reason for you not to stop.
So Section 170 (3) then applies: "If for any reason the driver of the mechanically propelled vehicle does not give his name and address under subsection (2) above, he must report the accident."
That has to be read in conjunction with Section 170 (6): "To comply with a duty under this section to report an accident . . . the driver
(a)must do so at a police station or to a constable, and
(b)must do so as soon as is reasonably practicable and, in any case, within twenty-four hours of the occurrence of the accident.
http://www.legislation.gov.uk/ukpga/1988/52/section/170
As I read it, you're in the clear!