We are taking a river cruise in France in April 2016 - want to extend our tour to London for 3 - 4 days after. That particular cruise does not offer the extended excursion to London - any ideas of who we might use to book?
I'd suggest doing it on your own and taking some London Walks tours or day trips.
If you are looking for a hotel/independent travel package, Globus Monograms has some options:
http://monograms.com/Booking/Monograms/Product.aspx?trip=5ML
You could do it quite easily yourself - buy the transportation from Eurostar for the 2 hour (and a little bit) easy ride through the Channel Tunnel, buy up to 6 months out for the cheapest fare, and use Rick Steves' London 2015 or 2016 book for hotels, what to see, and transportation; or ask a few questions here and we will do what we can to help. And read over the other posts in the England Forum, where you are with this question. Go back several pages and you will see all sorts of interesting threads.
You will save loads of money, and with decent preparation and study, London can be loads of fun and not too difficult.
Need a little more info. Where do you get off the river cruise? Where is your return flight to the US? You really don't need to have anyone arrange this for you. Get to London, use a convenient hotel, and take advantage of many walking tours, and the extensive hop on/off business. For the center of London, it is best done on foot.
1) Book a train from your cruise end point to Paris (hopefully a TGV service).
2) Decide if you want extra time in Paris, and book a hotel.
3) Book Eurostar train - Gare du Nord to London St Pancras.
4) Book a London hotel.
5) Have your departure flight rebooked out of London Heathrow.
6) Countdown the days. Make sure your passport is in order. Pack. Enjoy!!