You can fly to London (and several other cities), and from there to the IOM 365 days/year. During TT week it will be a lot of hassle (expensive) to get any vehicle onto the Island. If you really want to see the TT do you like camping? Foot passengers on the ferry from Heysham, Liverpool, Dublin or Belfast are not expensive. From experience, foot passenger ferry tickets from Belfast are available right up to, and during the TT.
Taking a bike to the TT is fantastic, but nowadays, during race week, the circuit is closed most of the time due to accidents. Public transport is very good and means you can drink more!