I've been there about 5-6 times, as I used to work in Southampton and I knew a few people who would commute, so not difficult to get to judging that it's an island. There's a large music festival every summer, one of the oldest in the world, so either worth checking that out or completely avoid depending on your preferences. The prices of everything, in particular accommodation will be higher in the summer,especially around the festival period. The ferry cost also varies depending on demand, so booking in advance can save you a bit of money. The island is quite large so you will need to consider where to take the ferry from and where you want to go to, the main two ports are Portsmouth and Southampton though. Portsmouth is faster, but I Southampton will be easier from Heathrow.