After you've seen Stonehenge, take a look at Avebury. It's similar in that the stones are aligned for various astronomical events, but the stones are not dressed as neatly as the Stonehenge stones, they're not stacked the way the triliths are in Stonehenge, and a whole village (Avebury) has grown up over the years in and around the stones.
Lon Las Cymru (the Welsh National Cycle Route) from Cardiff to Holyhead is an excellent tour, the off pavement options were great wqhen I did them. Llanberis Pass is also a fun ride, there's an excellent hostel at the top of the pass.
If you haven't bought maps yet, the Goldeneye maps are a really good scale for cycle touring.
I don't know what Scottish weather is like in September, but I thought the west coast of Scotland was great for cycling, the further north the better.