Yes narrow it down allot. Perhaps Just go and see London and Paris maybe a third city. But you will want to spend at least three days in London and at least three in Paris as well.

London and Paris, plus a day trip from each place will be plenty for 10 days. However, working on the theory that you can actually enjoy a totally exhausting whirlwind for 10 days since you can recover afterwards ….. You could add another city. Possibilities:

Amsterdam or Brugges — Leave London early one morning for Amsterdam, leave Amsterdam late the next evening for Paris). You’ll probably have to fly from London to Amsterdam.

Edinburgh — take overnight trains both ways (to/from London) and spend 2 days there.

With ten days, I wouldn’t do more than 1 country or two cities. Paris is not a city you see on the fly- neither is London. For 10 days, your best bet is to stick to just paris and london. There is plenty to do in and around the cities.

You didn’t say when your trip will happen. Regardless, you should be looking into Budget airlines – I don’t particularly care for RyanAir; I do like EasyJet. I flew them (EasyJet) London (Luton) -Madrid and was quite satisfied.

I did a trip like this once. Excluding travel time to and from, we had about 9 days.
We flew into London—then flew to Paris, spent several days there w/day trips out to places like Fontainebleau.
We then flew back, and spent the rest of the trip in London.

Then why did you purchase roundtrip tickets to London? Madrid, Barcelona, Milan, Rome, and several other Italian and Spanish cities do have airports, you know.

Why not explore Britain? Paris can easily be reached by Eurostar train, and makes an excellent 2-day excursion from London, but aside from that, you should spend more time in Britain. I think a great itinerary would include points of interest in England (including smaller cities, and the English coast [Cornwall is one good suggestion]), and a trip north to Scotland would be great to include in a British itinerary. If you have time, a short excursion to Paris by Eurostar would be great too.

Again: avoid a big-cities-only itinerary.

When is this trip?