I’d definitely go for hiring a motor. You’ll move around a lot easier and under your own speed. Trains in this country are overpriced.
As you’ve mentioned Stonehenge a couple of times, I’d suggest visiting there and branching out, as there’s lots in that part of the country to visit. It’s also very beautiful around there.
You have the New Forest national park to the south, plus the Isle of Wight.
To the north west of Stonehenge you have Bath, with its famous natural spring waters and Roman architecture/baths.
Close to Bath you can drive to Bristol and then on to Cardiff, which is worth visiting. Has a nice castle and is in Wales.
Or you could go south west to Cornwall from there, which is probably the prettiest part of the country, in my opinion. There’s lot of lovely little fishing villages to visit.
@Stretch616 likes to move around the country a lot and can probably add a few things to consider.