I am not sure how it is done in the UK (and it is certainly wayyy different in the US, especially in regards to phraseology), but in Germany you indeed do switch frequency immediately after departure - one notable exception of this rule is Munich.
But, and here comes the easy part -
it is always written in the charts how you are supposed to do it.
So, not much to worry about.
If there is indeed a departure frequency differing from what is written in the charts (when the departure sectors at Frankfurt are combined, for example), it will be in the ATIS.
And in the clearance.
Which makes it amazing to see that quite regularly a few pilots will STILL not get it.
But that is a whole different topic, I guess.
Note that in some cases the Tower controller still wants you to stay on his frequency, despite what is written in the charts - but then he will tell you.
Regards,
Robert