A user on Twitter mentioned that they could see Berlin Brandenburg as well as Geneva in the drop down menu when you select Southampton as a departure airport on the easyJet app.
Checked myself and it is true:
Southampton also appears in the drop down if you select Berlin as a departure although nothing is bookable (S20 dates are selectable but that is all).
One interesting thing (and I really don't want to stoke the fire of this tired old debate) is that the same thing shows when you select Bournemouth as a departure airport. Have tried a couple of other UK airports that aren't bases (Inverness, Birmingham) to see if it is a glitch causing Berlin to appear available from all airports in the engine, but it doesn't show from them.
Could be nothing, but may be an interesting one to keep an eye on...