I agree with the rest that seven countries in three weeks is definitely too much. I am going to Croatia for three weeks and I am having trouble fitting in all the nice places into that period. So my advice is to make a selection! A three-zone Interrail-ticket is definitely too much. Italy alone has enough places to see for a few months. As an alternative, you could visit northern Italy, Slovenia and Istria. Or start off in Geneva, visit Switzerland for a week, then go south to visit Venice, Pisa, Siena, Florence and finish in Rome. There are cheap flights from England to Geneva, Rome, Venice and Ljubljana.
Good luck deciding!