Early fall is a great time to visit europe and there is so much to see, that the most common trap is to try and do too much, and I would try to avoid visiting too many places. In Great Britain Scotland, Cornwall, Lake District are good places to visit, and of course London. In France next to Paris the South is very nice: Provence, Cote d'Azur (which is a bit touristic), and Tuscanny in Italy is fabulous (Florence, Lucca, San Gimigniano and many other places, small villages with fantastic landscapes, food and wine). Of course you can choose any other country of Europe, and you will have a fabulous holiday in Slowenia, Switzerland or Germany. Make a max of two countries and buy a travel guide to choose what interests you most. You cannot fail.
Have a nice holiday.