Given that Britain and France were at war almost permanently between 1792 and 1814, it does seem strange that Napoleon Bonaparte made no effort to occupy what were almost exclusively French-speaking islands just a few miles off the French coast. The harsh truth is that it was never really worth his time to make the effort.
The islands had long had a strategic importance in naval warfare that far exceeded the intrinsic worth of the farming produce of the islands. Earlier in the 18th century, Britain had fortified most of the harbours with gun batteries, garrisoned the islands with infantry, and naval ships were frequently on hand. Moreover, the local militia was well-drilled and could put over 3,000 men into the field at 24 hours’ notice.
King Louis XVI had ordered a French invasion of Jersey in 1781. About a thousand men got ashore on the night of 5 January, and next morning assaulted St Helier. They were attacked by the Jersey Militia and men of the 78th and 95th regiments. The French were utterly defeated.
With this example before him, and having no real interest in naval matters himself, Napoleon simply was never interested in trying to occupy the Channel Islands.
Answered by Rupert Matthews, historian and author.