The Imperial War Museum in London is excellent and has a relatively new exhibition about the holocaust.
The already-mentioned WWII museum in Gdansk is large. I spent about 20 hours there, watching every bit of English-language or English-subtitled video. On one of my visits I ran into a bit of a delay at the ticket counter, so those on tight schedules might want to buy their tickets online. As far as I know, there's no need do that days (much less weeks) in advance; I'm just suggesting it as a way to avoid standing in line.
If your historical interest happens to extend to the Cold War era, as mine does, I also highly recommend the Solidarity Center in Gdansk. It was a full-day stop for me. A lot of the capitals formerly behind the iron curtain have museums/memorials related to Cold War events and suppression of human rights during the period of soviet influence. This is especially true of Berlin, which has many such sights, but there are also interesting possibilities in Budapest (House of Terror, which sometimes has a ticket line), Prague (Museum of Communism), Warsaw (multiple sights), and even Timisoara (Romania).
Traditional guidebooks will not necessarily mention the secondary sites you may want to include. You'll uncover some by following links on the Internet. It's worth checking the InYourPocket website to see whether those folks have downloadable guides to cities you plan to visit; they may tip you off to sights not widely mentioned in tourist literature.
I want to second the suggestion that you not plan a trip that is solely focused on the war and the holocaust. I've been to a lot of the places you will want to go, but they have been spread out over four trips totaling at least 9 months, so it's not like I was dealing with the grimmest aspects of modern history every day. Even so, I still think of my 2018 trip (over 3-1/2 months in former iron curtain countries) as my "man's inhumanity to man" trip.