Hi Bill
I quite often pass Avebury on summer rides. There's not a great deal to see, unless standing stones are
really your thing. A good little pub/restaurant, but it only has a tiny car park and is usually jammed solid with foreign tourists. Can't even squeeze a bike in most of the time, and further car parking (in a field from what I can see) leaves you a bit of a walk to get to the stone circle. Most of the best roads for riding are to the south and west. If you head north, it's a good one, but you pretty soon hit Swindon, easy to get lost there if you don't know it.
Canterbury is a nice little walled town - the cathedral is stunning and well worth a visit. Some good little restaurants that I don't recall being too expensive, a nice walk around the town (only small, doesn't take long) and some interesting historic buildings as well as the cathedral. There's a car park nice and central, and usually a spot to lock the bike up.
I've done loads of castles and historic houses/parks in the south east, there are some very good ones, mostly Kent/Sussex, if you're interested give me a shout.
The others on your list I haven't visited myself, but hope this helps.
PS Blenheim Palace not far from me is worth a visit - large parklands, the palace itself is the seat of the Dukes of Marlborough, and was Winston Churchill's childhood home. A great walk around the park, with monuments and follies, Italianate water garden, lakes and woodland. The palace is stuffed full of exquisite paintings, period furniture etc, and there is a half decent restaurant/café.
Canterbury: