When sending mail to the Island from elsewhere, the Isle of Man is treated as though it were part of the United Kingdom, and mail from the UK continues to be charged at Royal Mail's UK inland rates. However, postcodes were not introduced in the Isle of Man until 1993, when the Island was postcoded as the IM postcode area as an extension of the United Kingdom postcode system.

What's the best answer to this question: is isle of man part of uk for postage?
yes