Strictly speaking the Isle of Man is a Crown Dependancy - the Queen of England is also Lord of Man - the regalities were bought back from the Atholl's (who had inherited from the Stanleys who acquired them in 1405) in 1765 - the stanley Lords were vassals to the EWnglish crown thus the English parliament reserved the right to make certain laws re the Island though internal laws were made locally.
The relationship with the UK (the Island is not part of nor ever has been) is a developing one - you should look at the IoM government website - www.gov.im for details