Four day mobile Eurail Flex pass Vs. London to Paris Eurostar tickets

I would buy point to point tickets because those automatically come with reserved seats (over and done) on the certain trains (e.g. Eurostar, TGV or IC trains)where you would have to add a step pay for a reservation when you are using a Eurail pass. And it is always possible that no seats will be available on those trains at the time you want. Also many people that have bought the Eurail come to this forum often confused after either not reading all the details or from a lack of response from Eurail to questions they have it seems . Not many seasoned travelers here use the Eurail passes.

what dates are you talking about? The prices sound high, with a little planning. Are you going First Class?

Eurostar passholder reservations are both scarce and expensive, so you are better off with point-to-point tickets!
But unless your trip is this month, $512 one way sounds very steep...are you looking at Eurostar.com for tickets?

Travel on July 25. This is the total for 2 adults and one child in standard class. Via raileurope.com. The tickets are even more expensive on Eurostar.com.

You are operating from an out-of-date, let's say post-war or 1960s, image of "rail passes." For the many trains that are "reserved seat only", and especially premium services like Eurostar and Thalys, having to (separately) book and buy seat reservations is a deal-breaker for most customers. Do you want to save (made-up number) $100 and have to take tickets on a 6AM train that you don't want to be on?

Even worse, waiting at a ticket window (with 25 people in line, most of whom can't speak the local language) because you can't board a last-minute decision train between major cities on the continent until you have seats assigned.

I am not following you, Tim. What are you suggesting I do?

Tim is telling you to forget Eurail passes.

Indeed that day is looking expensive for Eurostar. There are some slightly cheaper tickets later in the day, but not by much.
This said, Eurostar reservations cost $32.40/person with a Eurail pass, so that's almost $100 on top of the $525. With that in mind, I'd opt for tickets rather than the pass.

Thanks Carol. In addition, while it is convenient to just walk-on to regional rail when doing daytrips, there can be downsides. I haven't traveled in two years, but especially at rush hours and school hours, it is common to have to stand for portions of regional rail rides, on trains that are non-reserved service. I have succeeded in buying (daily) regional BritRail tix at machines, which eliminates the wait. But you have to pay attention to the conditions of sale. I mean, for example, rush-hour pricing, if you care about your ticket cost. I presume you get a fine for using a ticket that's not valid for the time you board the train.

It is already late to buy discounted Eurostar tickets for July, because they are priced kind of like air tickets - more expensive as the travel date approaches, and more expensive at hours of the day when Business travelers are more likely to pay a higher price for the same seat.

Don't forget the mandatory early check in for pre-boarding Immigration and Customs. I think this warning (and the time, 60 or 90 minutes) is presented to you when you buy the ticket. There have been reports of refused boarding even when the train has not left, because of the amount of "paperwork" for some travelers.

Thanks all! This has been very helpful. I will grab the point-to-point tickets and be sure we arrive early for the customs process.