What places have you considered as day trips from London? London is extremely expensive to use as a base. Have you thought of staying in London for a few days, then going to smaller towns to stay for the duration of your stay in England?
Why Zurich for three days? Again, a very expensive city and not that much to see.
Paris is expensive too, but not as expensive as London and Zurich. Plan on staying at least 3 days, and you’ll have seen the major things that you want to see.
Again, think about seeing (and staying in) smaller towns in France. Prices may be dramatically lower in smaller towns!
How will you be getting around? For example, will you go from Paris to Venice or Rome by air or by train?

I was in Europe in a lot of the places you are going in Nov/dec 07, it will be totally different when you are there because it was summer, but in relation to Rome, and basically anything in Italy, I would suggest getting a guided tour. Nothing makes sense without it. There is little to no signage and you can walk around for hours and not know what you’re looking at or how to get to the place you want to go.

If you’re going to Pompeii, I would definitely get a guided tour, otherwise it is just a waste of time.

Also, if you are going to Napoli, there is this awesome famous Pizza place, that only serve two types of Pizza, and they are only 4 Euro each, I can’t remember the name, but if you have the Lonely Planet Europe on a shoestring book it’s in there

I agree with Basie’s advice. But perhaps you have rented a flat or something?
If not, I do think it might be enjoyable to stay a few days in London, then venture out to other smaller cities and towns. Of course, transportation isn’t cheap either, whether in London, or out of it.
I don’t like spending less than 2 nights in one place, but I do think it’s good to mix it up a bit between big cities and small villages.
Zurich is boring and expensive IMHO. Venice is well worth it.
 I disagree about the need for a guided tour anywhere in Italy. If you read up about places beforehand, and/or bring a decent map and guidebook (especially one with Italian and Latin phrases) it is fun to explore on your own.