You have a maximum two travel days (00.00-23.59 CET) that you can use in your country of residence. On these trains you can board as many trains as you like. Hopefully you can find a connection from your home town that allows you to board the Eurostar the same day. 

Be aware that the pass holder seats on the Eurostar can sell out weeks in advance on popular departures.

If it is not possible to get the same day from Great Britain to mainland Europe you could ask interrail customer service to get an extra inbound/outbound day (you can only use 2 days of your pass in your homecountry). 

Keep in mind, that the Interrail Mobile Pass starts its travelday from 00:00 CET, this means from 23:00 GTM, maybe you can use a nighttrain to get to London. 

You should have no problems getting a morning train in the North West to London and connecting to a Eurostar mid afternoon or early evening. Your pass will cover you for all the necessary trains and the only cost will be a reservation for the Eurostar ( £30 or £38 first class).

UK trains can be reserved free of charge and you can use any train, unlike saver tickets which are usually after the peak. 

Note the transfer in London is not included. 

You should still have time on arrival in France or Belgium to catch an onward train if desired.

If you haven’t yet figured out the process of using a pass, you simply load each train you plan to travel on into your trip (in the app) and then just before boarding you transfer it from your trip to your pass and then you generate your ticket, which shows a QR code and a list of the trains for that day. The list will populate automatically as you transfer trains before boarding. 

If you make any reservations for trains with mandatory reservations you need these separately, ideally pre-printed, but often a pdf will be accepted.

For Eurostar you can check availability here (and book). I suggest you consider Lille or Brussels rather than Paris

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