Why Stanstead? It's 230 miles (370km) from London to the port to sail to the Island. Leeds Bradford International, or Manchester are much closer. The Island has its own airport - Ronaldsway. Can you book a flight to there?
Avis, Budget and Godfrey Davis are the big three names in car hire, camping vans are available from specialist hirers, but the Island is well equipped with many hotels and Bed & Breakfast establishments.
. . . And Kippers for breakfast is compulsory!!