At the school I used to fly from, it was mandatory to do a cross channel checkout if you wanted to take one of their planes to L2K, which was fair enough. The added advantage is that you have an instructor with you who will show you how to successfully do a flight plan, verify it is in the system, how to activate it (and remember you will need tell ATC your ETA for the zone boundary so work that one out on the ground first), how to handle different ATC authorities etc etc......
Of course, nowadays there is SkyDemon or EasyVFR that will greatly assist with all the planning and flight plan filing / GAR filing so its much easier.