Since it is mostly a middle-European event with climate-conscious attendees, most will arrive by train if using any sort of public transport. DBs connections to Hamburg, even from Berlin are abysmal if you compare to Leipzig.
- the distance from Berlin to Hamburg is 282km, there are 33 daily train connections, taking 1:44h or more
- the distance from Berlin to Leipzig is 148km, there are 27 daily train connections, taking 1:26h or more
The real question though is, why would you want to travel through Berlin under all circumstances ;)
Last time I checked any late evening connections Berlin - Hamburg were quite scarce. Maybe they upped it a bit then. Still, Hamburg being in the far north is not exactly great for travel.
Getting from Britain to mainland Europe by sea isn't cheap: ferries from Plymouth to Roscoff are usually over €200 each direction, no meal, no cabin, no car - one adult. And the ships are so massive that it cannot be for anything but artificial scarcity; I'd estimate you could fit about ten thousand people on one of their ships before it would begin to feel crowded.
An affordable ferry with a cabin would allow me to set off at noon, cross Britain from west to east, spend the night crossing the North Sea and arrive refreshed the next day in Bremen/Hamburg ready to use a Deutschlandticket or similar to continue by rail within Germany.
What I have chosen to do in the absence of such a service is purchase a €200 Interrail ticket for 4 days, reserve the Eurostar (Channel Tunnel) train for €60, and reserve an extra night at a hotel in Belgium for each direction. End result: a total of more than €500 and 36 hours travelling.
It's not easy being a climate-conscious traveller.
Maybe someone would like to build a climate-conscious travel search engine that includes all these routes.