help with wise planning of itinerary for paris and london

Welcome to the Forum, bhive

Is this for this year - a month from now? I'm a bit concerned with vagueness on dates, the good fares will be all gone, and the Eurostar is quite expensive if the tickets are bought late.

If net year we have a bit of time. Some tickets are released 6 months out.

Chamonix and Mont Blanc are in France and go into Italy. If you want Swiss Alps you might consider the Berner Oberland, particularly above the Lauterbrunnen Valley where the views are incredible. Problem is that the best places will be full by now - you are looking at peak summer tourist season.

Are you traveling on Space-A or jump seats?

Provence is a large region, so maybe you could narrow down one or two cities that you'd like to visit. I don't think that you could visit "Provence" during a day trip from Paris.
There are two major flea markets in Paris:
Marche aux Puces (Porte de Clingnancourt/ Saint-Ouen). Mondays are usually reserved for professionals, with shipping containers in mind, so go on Sat and Sun and be ready for an all-day affair. Better to take bus 85, which stops at rue des Rosiers and thus avoid the scam artists.
http://uk.tourisme93.com/saint-ouen-clignancourt-flea-market-access-and-opening-times.html

Marche Vanves. Open only on Sat and Sun, this might be easier to get to and to manage. Some people just like to look for "junque", so if that suits you, this might be where you should go.
www.fleamarketinsiders.com/brocante-vanves/

You didn't give your budget for hotels, so difficult to advise you. But you will definitely need an air-conditioned hotel in either Paris or London. Apartments in Paris do not have air-conditioning.
If interested in museums, you should buy the Paris Museum Pass - not the "Paris Pass".
Also consider purchasing a Navigo pass or buy carnets of 10 loose tickets for transportation in Paris.

London's transportation options are a little confusing, so you might want to ask for professional help on the London Forum. Also advice for hotels.

With only two weeks, consider sticking to Paris and London.

Switzerland is very expensive. We lived in Germany years ago and if we wanted Alpine skiing or hiking we would save by going to Austria and Bavaria.

We did two weeks in Provence and didn't see all we wanted. One day is not enough, plan on going back for another trip.

If you want to visit Chamonix, you probably need 2 nights there for one full day, or 3 nights for 2 full days. The train route from Paris means at least 2 transfers and 6-7 hours journey once you're at the Paris station. If you're flying into Paris and out of London, then see Paris, then Chamonix, then fly from Geneva to London. Or maybe start in London, then train to Paris, then train to Chamonix, then home from Geneva.

France is pretty big. To put it in perspective, the total area of Texas is 268,597 mi², France is 248,573 mi² - just about one ranch smaller :-)

If you fly into Paris and then out of London, you will have to pay a hefty tax.
Better to fly into London and out of Paris, or any other city you choose.

Fly in to Paris (6 nights)
Train to Annecy (2 nights)
Then Chamonix (2 nights)
On to Geneva/Lausanne (2 nights)
Fly to London (6 nights)