Broadway Tower, Cotswolds
Broadway Tower is a folly built on the ridgetop of the Cotswold escarpment (OS grid reference SP114362, map by Streetmap.co.uk) at the close of the eighteenth century by the Earl of Coventry. It is privately owned, but open to the public.
Following the ridgetop a few kilometres north is another fine viewpoint (or at least it should be, if I can ever visit on a nice day!), Dover's Hill (OS grid reference SP137397, map by Streemap.co.uk) above the village of Chipping Campden:
If you head a bit further North-East you should find Hidcote Manor Garden, yet another interesting National Trust property I've visited.