Spend some times in Paris then visit perhaps Brussels/Bruges/Amsterdam and then go to London. You really haven't given much information to come up with an itinerary. Have you been reading guidebooks? What are your interests and what have you learned that you might like to see and do between your time in Paris and London? Will you be willing to rent a car? Budget? Providing more info will get you better responses.

With two weeks you don't absolutely need another country at all.
Ditto FMT above--how can we know!!

It would also be helpful to know what time of the year. If in winter, stick to cities. If in May, visit the Loire Valley or Normandy between cities. You would benefit by having a car in the countryside.

First off, your entry and exit cities are both so rich in things to see and and do you could easily spend 10 days in each...

You WILL be back to Europe! Don't over plan!

I suggest 5 nights each in London and Paris as the bare minimum and 4 nights in your middle destination.

I would keep the third home-base close-by and not try to go too far afield. Some place quickly easy to get to via train would make the most sense. Amsterdam OR Antwerp OR Bruges are all good ideas - you could even do a day trip from any of these to another city like Gent or Haarlem or even to the countryside.

If there is a place more remote you really need to see this time (Venice? Prague? Rome? Munich?), then you absolutely want to fly there from London and then fly to Paris for the last part of the trip. Again, however, I suggest sticking this time to north of Paris and south of Amsterdam...