I work on the FIS service which is now at Swanwick. We use all three freqs together subject to workload. If it is busy we tend to split off the 124.6 (SE freq) which tends to be the busier. When crossing the Irish Sea via Anglesey or Cardigan Bay you are correct in saying that the correct freq is 124.75, however the transmitter site for the North freq 125.47 is on Snaefell on the Isle of Man and I would have no problem if you were to call me on that. Our most important concern is your safety particularly when crossing mountains such as North Wales or the Irish Sea.