The whole point here is understanding why we should ascertain the cleared level.
I'm sure i'll be corrected by people who know more about procedures than I do, but I believe it was to try to add some guarantee that an aircraft had meant to be transferred at that time. With the introduction of more and more standing agreements (for those who don't know this normally implies transfer of control on contact as opposed to at a geographical location) the chances of an aircraft taking someone elses call, for example, had much more serious consequences.
What irks me is (newer!) controllers who seem to stick to these rules without knowing why, take this example from a couple of days ago (also on ther seaford sector);
Aircraft departs Gatwick - No other planes within 20 miles.
Aircraft receives continuous climb from TC.
First call - "Hello London, speedbird**** heading 150 degrees passing FL155"
(okay, so he forgot to say what he was climbing to, but does it really matter? So he could have taken the wrong call - but is he clean? he's the only aircraft above that part of the English Channel!)
ATC response - "Speedbird**** roger, what is your cleared FL?"
"Oh sorry, Cleared, in fact levelling FL170"
So the aircraft levels at 170 and 10 seconds later begins his climb again!
This happens more and more because newer controllers don't seem to understand the book they just seem to know what it says.