So, apologies to any airliners over the south west of England this afternoon at about 16:00 local who also heard the call!
You don't owe anybody an apology. Your usage of 121.5 was entirely justified and reasonable. And just to be clear, 121.5 is a radio frequency, not an airliner frequency. If people get a bit anal about it (like the so called "Guard Police"), just ask for their call-sign. If the half wit does actually pass has call-sign, complain to his company via their Ops Director.
PM