UK AIP Entry
8.3 Flights Routeing via 6100N
8.3.1 All aircraft entering the Reykjavik OCA at or above FL 55 via 6100N at or East of 01000W from Scottish Domestic Airspace are required to obtain an Oceanic Clearance from Reykjavik OACC, as detailed in AIP Iceland.
Note: Flights routeing via RATSU (6100N 01000W) do not require OCA clearance from Shanwick OAC. Therefore, aircraft intending to route via RATSU (6100N 01000W) must not contact Shanwick Clearance Delivery.
Iceland AIP Entry
1.8.8.6 Requests for Oceanic Clearance
1.8.8.6.1 Aircraft entering Reykjavik Control Area shall contact Iceland Radio with their oceanic clearance request prior entering Reykjavik OCA:
1. From Edmonton/Murmansk Control Areas on HF frequencies.
2. Aircrews approaching Reykjavik airspace from the Scottish and Stavanger areas shall contact Iceland Radio on VHF 127.85 or HF frequencies to obtain their Oceanic Clearance. The Oceanic Clearance is transmitted from Reykjavik OACC to Iceland Radio once Scottish or Stavanger have coordinated the flight with Reykjavik, therefore the clearance should be available within the timeframe of 10 to 30 minutes prior to entry into the Reykjavik FIR/CTA.
AIrcrew that are unable to obtain their Oceanic Clearance prior to entry should seek confirmation of their authorization to enter Reykjavik airspace from their current ATS unit (Scottish or Stavanger). Having received such a confirmation the aircrew should enter Reykjavik airspace at the level cleared by that unit and continue attempts to obtain the Oceanic Clearance from Iceland Radio.
Independent of the above procedure for obtaining Oceanic Clearance, aircrews are reminded to maintain listening watch on the Scottish or Stavanger frequency until instructed to contact Reykjavik Control.
HF Frequencies and operational hours are listed in ENR 1.6.2.3.2
Hope that helps.