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Howard A Treesong wrote:Rare to hear a sonic boom near a built up area these days.

What sort of emergency does a helicopter let off that means that they scramble Typhoons to intercept?

If the helicopter were forced to come down in water, you would want to get eyes on it as quickly as possible to improve chances of rescue.

Or if the helicopter were entering controlled airspace without getting clearance, you would want to make sure it isn't up to anything malicious, which is what it sounded like here.

It said the aircraft were already on their way to the helicopter by the time the helicopter pilot realised he was transmitting on the wrong frequency and switched to the correct one.

It's standard procedure, though typically the "escorts" will intercept below the speed of sound. This was probably just a pilot error.


Edit:
Already on Wikipedia. http://en.wikipedia.org/wiki/Sonic_boom#2012_Sonic_boom

For anyone still interested, this sounds like it was a transponder issue, though I'm sure details will come out in aviation circles to make this a Teachable Moment (tm):

Such signals can indicate an aircraft has been hijacked or “gone rogue".

Squawking 7500 means, "I've been hijacked!" Apparently, aircraft in the UK squawk 7000 in uncontrolled airspace for VFR flights. So it's not hard to imagine that the pilot accidentally set the transponder to 7500 as he was transitioning to another airspace and got distracted before he could complete the task. Oops!

In the US we use 1200 for VFR flights (unless otherwise directed by ATC), which helps avoid these kinds of issues.