To be technical, that's too emphatic: it would have been highly improbable or, as a 19th century billiard player would say, a fluke shot (basically, accidental).

...speaking of pedantic. If I'm aiming at something and hit it, it's no accident. No matter how my TI claimed my shots were lucky or that I must have had help, every shot found the target I was aiming at -- it was no accident; it is not the aim for an accidental shot to have struck it's unintended target. If it was decided to continually fire AA over London with the hope that a shot took out a V2 and such happened, it would not have been an accident, however unlikely (and a huge waste of dwindling resources).
To elaborate, we are not disagreeing about your ad-lib point that there was no sure defense against an in-flight V2. Even with today's technology, however, it is not impossible for a rocket to get past anti-missile defenses any more than it's impossible for said rocket to intersect the flight path of a cannonball shot at the steeple of St. John of the Improbable (or whatever).