Itinerary

There is not enough information here to give you any good answers. How long is your whole trip? What is your budget for traveling between places? What "Sat-Thurs" are you going to be in Umbria (what month)? What are your priorities - what do you want to see or do? If you can fill in some of the blanks, we can help you figure it out a bit better.

I would seriously consider flying in or out of Rome, Venice or Milan. The cities you list are a straight line from London to Italy. This keeps you from having to double back to London.

I am traveling in October, leaving on a Thursday for 11 days, returning on a Tues. I dont have more time and a somewhat limited budget. Aside from the 5 days at the villa in Umbria, Austria is most important. The views are top priority, have always wanted to recreate Sound of Music scenes. Is one and a half days in Venice enough time?

Trying to understand your timing.
You fly on Thursday, arriving Friday in London. You have to be in Umbria on Saturday. How are you getting there? Then you are in Umbria until Thursday. That leaves you Friday - Sunday to do something, then use Monday to return to London (again, how?) to fly home on Tuesday. I am not sure how you could do Venice and Salzburg in three days, to be honest, considering that you also have to get to each place.

And if your trip is in late October, forget about those Sound of Music views. By that time of year, cloud cover and fog are more likely in the Alps than clear vistas.

I don't understand why you are flying in and out of London to visit Italy and Austria. With 11 days in Europe, you will spend too much time traveling from place to place and not enough time seeing the places you are in. Here's what your itinerary really looks like: Thursday - fly Friday - arrive London, see a little (jetlagged, tired) Saturday - spend the day flying from London to Rome, then train to Umbria Sunday-Wednesday - see Umbria ( 4 days sightseeing) Thursday - train to Venice Friday - in Venice ( 1-1/2 days sightseeing) Saturday - train to Salzburg Sunday - Salzburg ( 1 day sightseeing) Monday - Fly to London in the morning on Ryan Air (€55 now but the price will go up if you wait). ( 1/2 day sightseeing in London)
Tuesday - fly home. You'll have spent 7 or at best 8 days in total sightseeing. You will also have spent a lot of money on transportation.

If your flight arrangements have not been finalized, you should consider flying open jaw into one city and out of another. Unless you have business in London, you should fly into a city closer to your final destinations. But if you must go through London, EasyJet has flights into Innsbruck Wednesdays and Saturdays for about 40 pounds from Gatwick Airport. That puts you right in the middle of the Austrian Alps for very little money. You can easily get to Salzburg or through the Brenner Pass into Venice. From there, Umbria and Italy is an easy train ride.
If you have to fly out of London, you can catch EasyJet back to Gatwick from Pisa, Rome-Fiumicino, Verona or Milan-Linate inexpensively.

Along those lines, if you are going to fly into a major airport and then take a flight on another airline, plan to stay overnight before the flight. If your transatlantic flight is delayed, you could miss your onward flight and have to re-book (with a very expensive ticket) on the next flight out - and even have to spend the night anyway.