Personally,
 
I think that might be too much, either plan.  We just got back from a 17 day trip to Europe.  We spent 4 days in Southern Germany, 3 days in Venice, 5 days in Paris, and 5 days in Rome.  It definitely was a lot.
 
The biggest thing you have to remember is that travelling from one place to another, it definitely takes longer then you think.  I would plan for at least 1/2 day to travel between places, so if you go to 5-6 places, you’ll lose 3-4 days plus the days flying in and out!
 
If we did it again, we would cut out one place (probably Venice) and make more day trips from the city we were in.  Just my $0.02!

Dump Brussels because of time constraints.

You are better off saving your London days for just before you have to leave for your flight home.  Don’t want to risk missing your flight home.

If I were you, I would probably do London and Italy OR London, Paris or Benelux.  London has the benefit of being a hub for cheap flights to Italy (though if you are going in May, you need to book soon for a good price).   London/Paris/Benelux could be done by train (fly back to London from Amsterdam).

A general advice, know what is your butt in seat limit. As in, what is the maximum number of hrs you can handle being on a train?  I love trains, but my max limit is 4 hrs, I prefer to keep it under 2 hrs.  Night trains I avoid whenever possible.

If I were you……………….

London 2 days.
Chunnel it to Paris. 5 days.
Night train to Rome. 3 days.
Venice/Florence 4 days.
Fly back to London to finish off your trip.  buy your ticket Rome-London ASAP.

    Thanks for everyones input.  a lot of good info