You can use any slipway you want anywhere round the island, though it has to be said the ones in the harbour will be the best from the point of view of steepness and lack of exposure. The outlying harbours will be easier for leaving your car and trailer tho. Unlike the uk you don't need to get permission or pay to use the slips, though make sure you have insurance just in case.
When you book the ferry, bear in mind that they won't (don't like to) take trailers over 6m
.