So, on Friday 13 March, to get to Swindon, you should take a train from London’s Paddington station. The journey is 50 – 55 minutes and there are no changes to make on the way.
Next question is how long to plan to spend going to the White Horse, etc. I suggest 3 hours. That would seem to be a generous amount of time, you could do it in less, but with three hours you’ll not be rushed.
(You’d get off at Portsmouth & Southsea and take a taxi to the hovercraft departure point – you have decided to take a regular ferry), or you’d get off at the Portsmouth Harbour train station and walk a very short distance to take the regular ferry. There’s a ferry at 4.15pm, and the crossing time is 22 minutes, so that would be easiest/best, I suggest.
Return from Ryde to London
You want to be back in London by about 8pm on Tuesday 17 March.
There are ferries from Ryde at 4.47pm arriving at about 5.10pm, or 5.10pm arriving at about 5.35pm.
There are trains from Portsmouth Harbour at 5.45pm, arriving at 7.29pm, or at 5.59pm, arriving at 8.09pm. Both train require no train changes on the way. The 5.45pm train has a shorter journey time.
The 4.47pm ferry allows for an easy connection to the 5.45pm train.
Train Information Site : https://www.thetrainline.com/
Ferry Information Site : https://www.wightlink.co.uk/routes/portsmouth-harbour-to-ryde-pier-head/