Whilst Portsmouth is an island, access is good with 2 road and one rail bridge
Please use 3/A3 and M27. M275
If you have multiple cars that you don't need to use during your stay then you may find it easier to use the Park and Ride
If you have an electric car there are various charging points across the island and local to our apartments
There a plenty of public transport options:
Gatwick - there is also a direct train, that takes just over an hour.
Heathrow taxis can be arranged in advance and booked on-line. The cost is GBP 65 each way for a normal car. Aqua Cars 02392818123.
Southampton - and hour by train or 30 mins by taxi
The nearest train station is Fratton (about 1 mile). The fastest trains from London are from Waterloo. All Portsmouth bound trains stop there then and then go on to Portsmouth and Southsea and Portsmouth Harbour. You can check routes and book tickets through the Trainline
Arrive at the Hard interchange (Portsmouth Harbour Station) where you can catch a bus or taxi
From the Ferry port it is best to get a taxi directly.