Technically it is an absence from the UK.
The Immigration Act 1971, the law that governs leave to reside in the UK, does not define the term "United Kingdom", but the
Interpretation Act 1978 (which provides definitions for terms not specified in Acts of Parliament) states that the United Kingdom consists of Great Britain and Northern Ireland (i.e. not including the Crown Dependencies such as the Channel Islands and the Isle of Man).
The
British Nationality Act 1981, on the other hand, explicitly defines “the United Kingdom” as "Great Britain, Northern Ireland and the Islands, taken together" ("The Islands" are separately defined as the Channel Islands and the Isle of Man).
Therefore time spent in the Channel Islands is an absence from the UK for the purpose of ILR, but counts as time in the UK for the purpose of naturalisation.
Leave under specific parts of the PBS can be aggregated across both the UK and the Channel Islands (and the Isle of Man), but that is because the Immigration Rules make specific provision for that.
Also see
this Freedom Of Information request.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.