As long as you board the last train before midnight, it'll still count as one travel day.

Keep in mind that you need a seat reservation for the Eurostar London - Brussels : 30€ and it sells out days in advance so book early.

Brussels - Köln - Berlin costs 4,50€ on bahn.de. It's an optional reservations but very recommended for the cross-border ICE. (Do not take the Thalys to Köln)

Not possible to book seat reservations on all trains at once. Seat reservations are completely separate from the pass anyway.

- London - Brussels interrail.eu 30€ (2€ booking fee)

- Brussels - Cologne - Berlin bahn.de 4,50€ (no booking fee)

When doing Brussels-Cologne-Berlin, I usually do not reserve, unless I want to be in the quiet car or something.

Unless on busy travel days, like summer weekends or prolonged weekends during the year. On an average workday, it’s not really necessary.

Pro Tip: In Brussels-Midi, head immediately to carriage 22 of the ICE. There’s always plenty of unreserved seats there.

The other way around (Berlin-Cologne-Brussels), I usually reserve. The ICE Cologne-Brussels is usually quite full.

Wow thanks guys!! That's so helpful!! 

Just to doublecheck - even if I book each trip individually, so Berlin- Cologne; Cologne- Brussels; Brussels- London - as long as all these are on the same day they only cost one "travel day"?

Thank you, you are honestly the best

Indeed, on a travel day (0:00-23:59 CET, 23:00-22:59 GMT), you can take as many trains as you want. 

But if you plan to reserve Berlin-Brussels, do it in one go on www.bahn.com . Otherwise you pay another 4,5 EUR per seat. With Deutsche Bahn, you reserve for your enitre leg of travel, no matter how many times you switch trains (ICE’s, EC’s and IC’s that is).
If you want a later train in e.g. Cologne, you can tweak it by adding a stopopver, in the extended settings of the seat reservation planner (opens normally after clicking “seat reservation only”). Add an hour or two to the stopover time, and you’ll save a bit of time, enabling to take another train.

Like this:

It is worth remembering that a reservation is not linked to your pass, so you can easily change the train you eventually travel on by adjusting the trains in your trip. 

For example if you have an optional reservation for the ICE from Brussels to Cologne and see an earlier train about to leave with free seats you can always board that train after adding to your trip and simply forget your reservation. (If you have time you could even see if they would transfer your reservation free of charge).