An IRBM or ICBM without precision guidance and conventional ammunition is a waste of resources. To hit and destroy a bomber or other valuable target without terminal guidance of some kind, like lighting the traget with a laser is less likely than a hole in one in golf. The real danger comes when Iran develops working precision guidance systems.
The intent and the consequences can be political as well as military.
An IRBM impacting in the centre of London would have earth shattering political consequences and ramifications, both for the government and for the MOD.