Stonehenge is not worth while. It's a big pile of rocks. Historic, sure, but really not worth paying to visit. If you take the A303 road from London to the Southwest, you'll spend time sitting in a traffic jam past it, so you'll see it for free anyway. Avebury, with isn't far away, is free and more interesting IMO.
A couple of must-do's (though now getting quite popular/touristy/busy) are the Wild Atlantic Way in Ireland, and the North Coast 500 in Scotland.
The IoM is nice, and has a well developed tourist industry despite its small size. It still has stretches of road (mainly over the Mountain) that have no speed limits, and a combination of fast sweeping and slow nadgery roads. I'm sure you know about the bike racing side of it, which is like nothing else on earth, but realistically unless you've booked well ahead it's very hard to get ferry crossings and accommodation for the races. Still plenty to do and see though, start by looking at www.visitisleofman.com
In the rest of the country I'd recommend the Southwest. North Wales and the Lake District for great roads and scenery. You'll clock up plenty of miles doing all those. If you want particular historic sites or cities that's a whole nother other
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