Ashley, If you're referring to Rick's 21-day "flagship" tour, it begins in Amsterdam (actually, Haarlem, which is just a short distance from Amsterdam)
If you're planning to follow a similar route, I'd suggest starting in London. This will not only provide you with a "milder culture shock", but also allow you to get over jet lag and get sorted for the trip to the continent. You'll have to budget a bit more for the U.K., as it's usually expensive (depending on the exchange rates at the time).
From London you could either take the EuroStar to Paris for the next leg, or use a cheap flight to Amsterdam from Luton or Stanstead if you want to faithfully follow the RS tour route (and end in Paris). In any case, open jaw flights would be a good idea.
I'm never "disappointed" when travelling in the U.K. While it's somewhat similar to North American culture in many ways, there are still lots of differences. Be sure to review the "English language" section in Rick's books, as some of the expressions are very different. For example, DON'T use the phrase "Fanny Pack" in the U.K. or Australia!!!
Good luck with your planning!