Barf, you are right about Annex 10 relating to the use of BREAK. However, CAP413, the UK RT Phraseology manual (downloadable off the CAA website) says that BREAK indicates "the separation between messages". BREAK BREAK is still in use - theoretically - and indicates "the separation between messages transmitted to different aircraft in a busy environment."
As a UK controller, CAP413 is what I am supposed to adhere to.
I don't use BREAK very often but it's useful in certain circumstances such as "Read-back correct, break, Ryanair 449 enter the apron via Uniform, Stand 3."
This would happen when an aircraft has requested and received his airways clearance but I have just become aware that an inbound has vacated the runway earlier than expected and is eager to know where he goes next. (Why did I use Ryanair as an example?
)