Well, sorry about the loss, maybe airport staff took it and sold it to a pilot who didn't know better?
A preliminary announcement of investigation should be enough to set them apart. If they don't set them apart, but overwrite them, you can later sue them for destroying evidence.
Lets not get all sue crazy, this is England we are talking about.
Yes, you need more than that;
Part 1: The crash
Don't put much fuel in your aircraft, scream mayday, force an emergency landing by a river or sea or something, stay stranded for 9 hours, swim in the river/sea as if you were looking for food in the river but you didn't have your raft so you got swept away by the current, hold on to a rock till you get rescued (at this point, call a friend who is in on this and tell them to call the emergency services, let them tell the police they may know where you are so you can prepare for your big entrance to the press room), watch out for cramps of course, the steak you had when you landed may affect you.
Part 2: Post-Rescue
Act like its been the worst day of your life when you get rescued. Go to your press conference, tell them that a raft would have helped you so that you wouldn't have almost died. Tell them you plan to sue, and get the world outraged.
Scenario 1:
Some one fesses up. (hope not)
You sue the Special Branch for the raft (always overestimate the value, say its a limited edition, say it was sentimental), sue them for distress, sue them for everything you can think of (best route).
Simple right?
