Since you're only going for 10 days, I would almost recommend going to just those 2 cities, or maybe with one more in between. Big whirlwind European tours feel so hectic, and you're left with only a brief confused impression of each place you visit.
It might be easier to go to London and Paris instead, or Rome and somewhere else in Italy like Florence or Venice. London and Rome are pretty far apart -- 20 hours by train! It would probably also be easier to find a tour group that went to those sets of places together.
Sorry I'm no help on specific company recommendations -- I'm a DIY kind of girl, myself. But I do know that
Rick Steves has a lot of tours. I've never been on a tour, but his guide books have some of the best advice I've ever found. Check out the website at least for packing advice, too.
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