I've raised this issue a couple of times previously.
IMHO, the reason that 124.75 doesn't work right where it is printed on the chart off Holyhead is that the transmitter at Clee Hill is shadowed by Snowdon. The signal on 125.47 is good however and you speak to the same person, in my experience anyway.
I don't know what it takes to get a thing like this changed, however. It would be a simple matter to change the chart, but despite raising the issue directly with controllers at London info (terrific peole, by the way) the wheels of bureacracy grind exceeding slow and the current edition still has the misleading 124.75.
Perhaps the 'authorities' don't appreciate that it's fairly lonely out over the Irish sea when the military have gone home and Dublin have lost interest because you are in UK FIR. Putting the 125.47 on the chart would make a big difference for those who don't know about this problem.
In the past crossing from Arklow I've had to make big diversions around so-called '24Hr' danger areas because the military who are supposed to be responsible are in the pub and I didn't have the Northern England chart which does show the 125.47. I write it on the S. England chart in indelible pen now!