I found a post on uboat.net forums that provides some helpful context. I pasted it below but the question is still unresolved.
After the announcement of Germany's unconditional surrender the Admiralty made special arrangements for U-boats at sea. On 19 March 1945 the Commander-in-Chief Western- Approaches (Admiral Max Horton)had requested clarification on procedures and instructions for surrender of U-boats and their internment at Loch Ryan and Lisahally. (1)
In order to institute a uniform policy, Allied naval authorities needed proper guidance. Based upon a subsequent meeting at the Admiralty on 27 March, Horton suggested that German U-boats surface, report their positions, fly a black or dark blue flag, jettison ammunition and torpedoes and follow routes to selected Allied reception ports. (2)
With minor adjustments and alterations, the Admiralty accepted these proposals under the code-name 'Adieu'. Since German captains were likely to ignore Allied transmissions, instructions for U-boats, drafted by Horton's staff were added in two supplementary annexes to the special naval orders. (3)
Acting through German naval authorities, the Royal Navy hoped that captains would cease active operations immediately. U-boats were ordered to obey Allied orders without hesitation. At Admiral Burrough's direction, British naval authorities allowed German transmitters to continue broadcasting surrender instructions in code and plain language. (4)
The reason why they were allowed to broadcast coded messages was for the U-boats that were operating in the Far East (Monson boats).
(1) PRO, ADM 116/5202, Message from Admiral Max Horton to Admiralty, 19 1529A March 1945.
(2) NAC, RG24, Vol. 11,752 File CS638-3, 'Minutes of Meeting at the Admiralty concerning arrnagements for dealing with German U-boats after surrender, 27 March 1945.
(3) DHIST, File 1650-U-Boats-1945 Commander G.F. Todd, RCN, Naval Assistant (Policy & Plans), 'Third Interim Report on Plans of Surrender of German U-boats', 17 April 1945.
(4) PRO, WO.205/259, Message from ANXCF Main to Flag Officer Kiel, 09202IB May 1945.