No it won't work !

You need to book a reservation in advance or you won't be able to board

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

The last available Eurostar here is at 11:03. Does it not fit your schedule ?

There would later Eurostars between Lille and London but you then need to add 10€ for the TGV to Lille. Could work

No. I was looking at any time after 2pm. Also, how early do I need to be in the station if I look at 11:03am train?

The planner app is just to register the trains you are taking, it does not book seats.

Seat booking has to be done separately, either through the Eurail reservation website portal or through alternative sources which are explained here

For trains with compulsory reservations such as Eurostar you MUST obtain reservations or you will be denied travel or possibly fined.

In all cases you need to add trains to your pass through the planner app.

If there is none after 11:03am, even checking in the station will not allow me to book the train after 11:03am?

Edited my post. You could go to Lille.

1h earlier will be enough. Last time I passed customs and everything in less than 10 min (okay it was a weekday).

It is worse departing London in my opinion.

Edit : No you won't be able to take later Eurostars. They sell out weeks in advance and have limited passholder fares

There is a limited number of pass holder seats on the Eurostar between London and mainland Europe. 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

If you don't get any result, 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

Thanks for the Lille option. So I can look at travelling from Paris to Lille to London. All on the same say using my Eurail pass?

Your Eurail Pass is valid. You'll need 2 reservations though :

- Paris-Nord - Lille-Europe (or Lille-Flandres) 10€

- Lille-Europe - London St. Pancras 30€

An hour between the 2 trains will be enough for customs but you could explore Lille for a few hours :)

If the TGV arrives in Lille-Flandres, add 10 min to walk between the 2 stations

You would have to take a different train Paris - Lille (French TGV that would also need a reservation) and leave enough time to check-in for the Eurostar departure from there to London.

Lille has 2 stations, London depart from Lille Europe, many of the trains from Paris arrive at Lille Flanders, they are less than 10 minutes walk from each other.

So, I should look at these two trips separately in eurail and not look at Paris to London via Lille?

As you prefer. Maybe check each leg separately so you'll see all the available TGVs between Paris and Lille if you want to stop there for a while

This is great advise. Thanks a lot.