One of the most reliable places to hear the sonic boom is along the south Irish coast between Waterford and Cork. Concorde tracks west from London, over Bristol, and goes supersonic as soon as it is out over the sea between Ireland and South-West Britain (hence references above to hearing it in North Devon). In reverse is slows down at this point just as it reaches landfall.

Although you get the full sonic boom directly beneath it's path even from its cruising height (hence the problem), the noise tails off to either side the further you get from the aircraft. The sonic boom can be heard, softly, for many miles to either side of it's track. Atmospheric and weather conditions on the day determine how far it travels, and which way. On land, if you didn't know about it you probably would not notice it at all.

Mary2e:

Taking off from airports that is not the sonic boom from the aircraft, but just the noise from the engines, in the normal manner. The sonic boom only occurs when the aircraft is flying supersonic over the sea.

Skylink USA:

Those of us who have heard the "Old Girl" pass overhead London over the years know her distinctive "rolling thunder" sound. It's a lower frequency noise, quite different to normal jets. It's the signal for me to go out into my back garden and watch her go over in the distance.