You have two days to get in or out of your country. Usually that’s the first and last day of your trip. On such a day, you can take as many trains as you want to get out of the country. You can also get to a ferry port, or airport, bus station… Leaving doesn’t necesirally need to be by train.
A pass day is valid from 0:00 unit 23;59 (but you can finish a trip that started before midnight, until after midnight, like your trip with the Caledonian sleeper).
But to get out of London with the Eurostar, you will need an extra pass day if you want to do that by interrail. So you’ll use up both days to get out of the country, so there’s no way for you to get in the country any more by train with your pass.
Since it’s for next summer, you can book well in advance and score cheap Eurostar tickets of +/- 59 EUR, comparable to the cost of a seat reservation and pass day (30 EUR +cost of a pass day) to avoid losing two days on one outbound trip or book the sleeper tickets and use the pass days on the Eurostar.
Depends what makes more sense to you cost wise :)