Theo: late fees on registered letters were higher than for non-registered -- often substantially so -- because of the extra paperwork and direct staff attention required at a time when the clerks were rushing to get the mail prepared for dispatch. There were in essence two separate late fee systems for registered letters -- one for London, and one for everywhere else. Presumably this was due to the high density in the capital of both population in general and businesses likely to send lots of registered letters and frequently cut it fine when doing so. For provincial offices, a 4d fee was introduced in the early 1860s, which is the one Austin Davis mentions, and gave an extra half-hour as I recall.
Additional stages that applied only to London offices came in from the late 1870s, first at the chief office in St Martins-le-Grand, then in the other big central offices in the main business district, and then in other branch offices in the capital. The fees could go up to 1s. For a rundown, I've attached a scan from the page in my 1908 Post Office Guide (circa p590) that gave the arrangements for posting registered letters in London. As you can see, the late posting arrangements were quite extensive, and would apply to insured letters as the British PO automatically treated them as registered.
Full details can be found in Parmenter's "
London Late Fee and Too Late Mail 1840 to 1930" (registered mail is only a small part of that, obviously). It's a hardback, about 150pp, and only cost £10 from Vera Trinder. There are also a few notes in the introductory chapter of Moubray if you have that.
I did mean to pen an article looking at registration late fees for the GBJ, but have never got around to it. Maybe I will!