you have to figure the mimimum you would want to spend in any city is 3 nights and cities like Paris you need 4 nights at least. You could train from London to Brugge or Brussels...spend 1-2 nights...train to Amsterdam...spend 3 nights...train to Paris...spend 4 nights. Rent a car and drive south through france and northern Italy ending in Venice...fly back to London. I have found a lesser drop off fee when you rent in France and return car in Italy then other countries. It would give you approx. 6 nights to drive from Paris to Venice...or you could take train from london to Paris, train to Brussels/Brugge...train to Amsterdam...train to Berlin train to Munich...train to Venice. ...Depends on what you want to see the most.

Since you lived in Europe for 5 years, you must surely know more than many people here, and certainly more than me. That said, take a look at some of Rick Steves 'Best of Europe in 22 days' itineraries. One covers Amsterdam,, Munich area, Venice, (Rome and Florence but you're doing Brussels and London so delete) Paris. Note there is no mention of Madrid or Berlin.

If your bride to be loves rally drives, this is the trip for her.

I'd say one week each in London, Paris, and Venice. If you absolutely insist, add in one more: A'dam or Munich. That's still a lot of intercity travel (train or plane) plus packing and unpacking.Try to arrive in one city and fly home from the last one, even if a plane change is needed.

If you're feeling ambitious you can do a daytrip or two. There are still plenty of places to get lost in that can still allow you to get back your hotel by the end of the day.

If the weather is good (when are you going?) you could do a drive to the Champagne caves, even stay overnight.
Or go down to the Loire valley and visit the chateaux.

Degas is SO right~
These days, the Yankee and I grab the keys and run for the car, whoever gets there first gets to drive. The other has to keep their mouth shut, unless collision with a large truck is immenent. He never drives fast enough, or too fast. I scare him to death.
But we have been married a long time, so it is no longer grounds for divorce.
IF! I had been taken on a honeymoon of this sort...Scarlett would most likely not be married to the Yankee anymore.
Do both of you a favor, take the honeymoon together, not as driver and passenger. Use those hours to make love and start your lives together, not reading maps and trying to find that fork in the road!
Happy Honeymoon!!

if this is a serious question and you really want to do it.Then I have suggestions as I did a driving three week trip. First ditch spain (way too far away. do open jaws! I would do london(4 nights) train to paris(4 nights)Then rent a car and do amsterdam (2-3 nights) and drive down germany(i would stop in southern rhine ) to munich (2 nights),onto innsbruck and down into venice (3 nights).(plenty of driving and small towns. then over italy from venice up lake como(on extra night) and into switzerland (she would love it). You can return the car back on the French side of geneva with no drop charge!!dont know how many days you actually have but you will need to add the night in germany from amsterdam and a night in the alps and near lake como too, get a europe may and use a highlighter.theres nothing like a visual to help reducing driving time. plan what you want to do and see a page for every day. the more blitzy the trip is the more planning is involved!!! we found the we were getting tired by the last week and scaled down and skipped some things.it took me a year of planning to pull my trip off and I read lots of books to get a jump on the maps etc! good luck