Use of the two Oceanic clearance frequencies is based on where the aircraft is registered.
See NATS Document ENR 2.2 Page 22
http://www.nats-uk.ead-it.com/public/in ... id=11.html123.95
"For use by aircraft registered in States West of 030°W for requesting Oceanic clearance direct from OAC."
So North & South America (and maybe New Zealand - though it isn't mentioned)
127.65
"For use by aircraft registered in States East of 030°W for requesting Oceanic clearance direct from OAC. For this purpose Australia is regarded as being East of 030°W."
So Europe / Asia / Africa and Australia (which is mostly 'East' anyway)
(30W is basically mid Atlantic)
123.95 and 127.65 are both transmitted from the Waltham in the Wolds site in Leics - where both seem to have a positive offset (7.5k?); and doubtless several other sites too.
These two are used like the HF frequencies:
127.9
124.175
"To be used as far as practicable by aircraft when East of 020°W so as to reduce the loading on HF channels.
Traffic received on this freq is handled in exactly the same way as if received on HF."
(20W is basically the southern tip of Iceland)
So they are probably transmitted from the west coast of Ireland and/or Scotland (such as Mangersta on Lewis which is just about as far North West as you can go, apart from St Kilda).