The advice from the experienced travellers in the community is to wait with activating the pass until just before boarding the first train of your travel. So I recommend you to deactivate your travel days and pass. Please read more about that in my next reply.
Regarding seat reservation on the Eurostar, you should make those ASAP. Read more below.
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