Jersey is southernmost and biggest island of the Channel Islands group just offshore Brittany and Normandy in France. Despite its location near the french coast, the Bailiwick of Jersey remains a private property to the British Crown; therefore it holds an independent status inside the United Kingdom. The capital and main harbour is St. Helier.
The island is big enough for cars to circulate, and a maximal speed of 40mph could facilitate hitchhiking. The longest straight journey you could undertake are some 11 miles (about 17km), however. Condor Ferries serves Jersey from the UK and France, but those ferries don't seem to be hitchable (being paid for per passenger, not per vehicle).