You'd better check yourself the price of normal tickets. For Frankfurt - Brussels, check bahn.com. For Brussels - London and London - Paris, check eurostar.com.

Additionally, when you use a pass, the big question is whether there are still pass holder seats available on Eurostar. You can check that here:

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Availability for this month is not great, at best.

As I am sure you can do the sum yourself.

Simply compare the cost of a 4 day pass plus 60euro eurostar res fee against the train prices on operator websites Dbahn and Eurostar for your dates. You will also need to check Eurostar availability on Beurope for pass user availability.

Just remember that prebought cheap tickets are usually non changeable or refundable, where a global pass is fully flexible, other than for reservations.

This is helpful, thanks! I didn’t know where to start when looking at individual tickets. 

If you decide to buy a Eurail pass I strongly advice you to check the availability of pass holder seats on the Eurostar and make the reservations as soon as possible. There are very few, if any, pass holder seats available in the second half of  December.

How do I see eurail pass seat availability? I can see the train times in the eurail app and the option to pay to reserve a seat but without having a pass already I’m not sure how to see how many seats are available. 

The Eurail app can't book any reservations. It just forwards to some of the websites where you can book. Instead, please check the link I posted. Here it is again:

https://www.b-europe.com/EN/Booking/Pass#TravelWish

You don't need a pass to check availability there.

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

If you want to do Frankfurt-London, you need to pass Brussels anyway in most cases. You might want to check the prices entirely on bahn.com. Deutsche Bahn has a deal with Eurostar, which can turn out quite advantageous than booking them seperatly (one on DB the other via Eurostar).

For example, I checked prices on 15 December. Last ICE from Frankfurt to Brussels, first Eurostar out of Brussel to London on the 16th. Eurostar prices are 40-50 EUR cheaper than booking it seperatly on Eurostar!

You might want to tweak the settings a bit, leaving late in Frankfurt, adding plenty of layover time, having enough time in Brussels for a night and a short visit!

When do you want to travel?

Other Eurostar tips: weekday trips are usually cheaper than those on Friday or Sunday. Make sure to be well in advance at the Eurostar terminal 1h to 45 min for passport checks and some secutainment.