It would appear that the surviving stone windmill towers at Easton on Portland are amongst the earliest surviving windmills in the country. They are recorded in a land record on Portland of 1608 and are probably somewhat older (ie C16th).  

Early Stone Tower Windmills 

The first stone tower windmills appeared in northern France c1290 with the earliest in England near Dover Castle c1295.  

The Dover site is thought to have been on high ground about 250 feet SSE of St Marys Church and is shown on a plan of Dover Castle (History of the Kings Works). There are building accounts of its construction which started on Monday 29th November 1294 when Nicholas de Eynho, the master mason was paid 3d a week for constructing the foundations. The records continue with six masons served by two women and nine men labourers constructing the tower etc.   The records show that it was built in stone so could have only been a tower mill ......   "in uno molendino ventrico de petra de novo construendo in dicto castro"  (Source : Pipe Roll 29 Edward I rot.22 mem ii - noted in Journal of Archaeological Association 1966 p75 "A note om early tower windmills by John salmon BA FSA)