Road trip

Are you thinking of renting a car in Spain and driving it to the UK? EXPENSIVE drop off fees.
As a compromise, take the train from Barcelona to someplace in France to rent a car, like Montpellier. Then complete your trip as far as Paris and drop it there. Car will be useless in Paris. Then take the Eurostar to London, another place a car would be useless, let alone one with left-hand drive.

Don't forget to get a Swiss vignette sticker for the car when you enter Switzerland. Get an International Drivers Permit as a legal translation of your drivers license.
Lastly, watch out for ZTL's in Italy, and speed trap cameras everywhere.

Are you using a car?? And how many nights are planning on being gone?

Do you mean you plan to day-trip to Venice from Milan? I would definitely not do that. It will mean 5 hours on the train (more than 6 hours driving, plus irritants like parking). Skip Venice until you can actually spend some time there. Trying to cram Venice into just a few hours in the middle of the day will subject you to the worst of the tourist throngs around the most popular sights, like San Marco and the Rialto Bridge.

We will be traveling about 15 days. We will be renting a car.
Do you think a day trip to Venice is wrong? No judgement just curious. Our family loves exploring the towns, not a big fan of all tourist things.

It's a very, very long drive from Milan. ViaMichelin is more conservative than Google Maps and estimates about 3-1/2 hours each way. Plus 50 euros in tolls round-trip, and whatever you pay for fuel.

What you're considering is roughly equivalent to staying in Baltimore and deciding to make a day-trip to NYC. Instead, I would suggest trips from Baltimore to Philadelphia or to Washington DC (though each of those is obviously worth multiple days). From Milan, I would suggest Bergamo as a nice destination for a day-trip.

Keep in mind that when you get to Venice, you will be moving around either on foot (over hump-backed bridges) or by boat. There are no buses. It's not like you can hop off the train and see a bunch of key sights rapidly.

Venice is one of Italy's (and Europe's) top destinations. I'd be madly frustrated with just a few hours there, and I am always disappointed by posts from people who spent very little time in Venice and didn't like it because it was so crowded and touristy. That's what I fear will happen to you if you are just there for a few hours in the middle of the day.

That's about 1300 miles, 23 hours in a car. And that doesn't include London.

Is there any way you could eliminate
Italy and just do France and Switzerland and London.

Just seems like a lot of travel.

I think to Milan is fine. To go to Venice, take the train on a Saturday and get a 2 for 1 Saturday Special Freccia price for 90 EUR for 2. Leave at 7:15 and return at 7:50 pm gives you 10 hours in Venice. Will Venice be crowded? You bet!

It's not as accurate as using Via Michelin, but looking at your potential time in the car, Rome2rio will give some hints as to routes, how long they will take to drive and how much driving might cost.

For example, Milan to Zermatt is about 3.5 hours driving time through mountain roads. Hopefully this trip is for the summer. And by the way, you can't drive all the way to Zermatt. You have to park in Täsch and take the train up.

And, Zermatt to Paris is about 7.5 hours driving time.

I highly recommend that you go to Gemut.com, download and read the free brochure on renting a car in Europe, and call them with your plan. They will price this out for you, including any drop-in-a-different-country fees. You should probably be sitting down for that.

I'm no stranger to road trips. I lived in Germany for 3 years in the 80's and navigated while my husband drove in 8 countries from 2009 through 2013. We love road trips. But, pay attention to what the Gemut guys and others say about costs and what is realistic in terms of time.

A road trip in Europe makes sense if you want to see the countryside, visit villages, small towns or castles along the way. If you just race from one big city to the next take the train instead.

I agree with Martin. You are planning an trip by car that is best suited for trains or planes. Either pick the destinations you want to see and then pick the most appropriate mode of transport (in this case a combination of trains or planes) or pick you mode of transport and build your trip around that. If you want to do a proper road trip in Europe, then focus on a smaller geographical area, visit small towns and the country side and use the smaller roads, where the car is an advantage, not a burden.

Thank you everyone