There are very good connecting routes via the M3 from Reading to all three of the mainland ports serving the Isle of Wight in Southampton, Lymington in the New Forest and Portsmouth.
Driving will take under 1 hour 15 minutes to Southampton Docks for the red Funnel services; just over 1 hour and 20 minutes for Portsmouth and a little more than 1 hour 30 minutes to Lymington Pier where the Wightlink car ferry services depart. Hovertravel also leaves from Southsea which is the same journey time as Portsmouth. So the right service for you will probably be dictated by where you are travelling to on the Isle of Wight. Whichever route you choose, you will be pleased to know that it will be more than 3 hours quicker than the fastest journey time by car to St Ives in Cornwall. So you gain almost another full day that would normally be lost in travelling time and you can be boarding your ferry before you would have past Salisbury heading to the West Country.