Giles, it has been common practice at LTN for quite some time, but is not a "norm". LTN as particularly smart and flexible ATC, in my opinion. The only scenario it is used, though, is where the first needs full length or isn't quite read, the second has informed TWR that they only need intersection and are ready, and there is a big enough gap in the arrivals for both departures. Even then, some controllers won't do it unless the second aircraft (departing first) is tight on a slot.
I't's a very good system, and as long as neither pilot wastes time, the first entering/second departing won't even have to wait to advance the thrust levers after turning around.