What is the nationality of people from the Channel Islands?

They are British as the Channel Islands are Crown Dependencies of the UK. Residents of the Channel Islands are Britons, though the islands themselves don't enjoy the same benefits as the UK proper. Actually, considering that the wide majority of the residents are quite well off, they might not see that as a drawback at all. My mate was married to a woman from the Channel Islands. Her father owns 19 pubs located throughout the archipelago. The man has more money than God. 

They are (U.S.) Americans. I assume you mean the Channel Islands of California, which include the town of Avalon (mentioned in a Crosby Stills & Nash song), right?

Currently and legally they are British. 

The question is not easy to answer in that form, the inhabitants of the Channel Islands have many nationalities, British, Portuguese, Polish and many more. Only 50% of the inhabitants of Jersey (for instance) were born in the island. Channel Islanders are considered to be British, they are nothing to do with England per se.

Their inhabitants are British citizens. The Channel Islands were owned by the Duchy of Normandy, and passed to the English Crown when William the Conqueror became King of England in 1066.

Channelliers.  Not really, but sounds good.

Islanders as a generalisation, Jerseyman/woman from Jersey and Guernsian from Guernsey. I do not know what they use from Alderney, Sark and Herm.

unitedkingdomandnorthernirelandese

commonly pronounced as "british"

they are remote on the channel islands lol