Joel - You get a gold star!
Bob - Since you will be traveling on several high-speed trains for which last-minute tickets can be fairly expensive, if you wait until you get to Europe to buy tickets, you will probably pay more than you would for a railpass, even adding in the reservation fees you'll have to pay for trains that require them. However, if you can commit to specific departure dates and times and book online well in advance--up to 90 days for Thalys trains (Brussels-Amsterdam and Amsterdam-Paris) and for TGVs in France--and even longer for the Eurostar for Paris-London, you can get discount fares that will definitely be cheaper than a railpass. One other thing to be aware of is that for Brussels-Amsterdam the fare for IC (Inter City) trains is always the same no matter when you buy it and it is cheaper than the standard fare for Thalys trains on that route. However, booking well in advance at www.thalys.com can get you a really cheap ticket for both Brussels-Amsterdam and Amsterdam-Paris. (Thalys is your only choice for Amsterdam-Paris.) Book Paris-London at www.eurostar.com. Book your trains within France at www.tgv-europe.com, using Great Britain as both your country of residence and the country in which you will retrieve your tickets. This will keep everything in English and keep you from being bumped to the Rail Europe site. Don't worry. As Joel indicated, you can pick up your tickets at any SNCF station or boutique in France by showing the credit card used when booking and showing the confirmation number you'll be sent by e-mail. If you get a PREM'S (discount) fare, you print the tickets yourself and take them with you.