I'm not sure I'm following just what your question is. Carry-on suggests one thing to me, and carrying a compass doesn't seem to make sense as being useful in the cabin under any circumstance I can think of. A whistle makes sense, as you can blow it if you're trapped in a cabin - whistles last longer than your vocal cords and are better heard. A flashlight so you can see and be seen if the cabin is filled with smoke or other vapor. I carry a Swiss Tech Micro, which is a pliers with screw drivers in the grips - it's made it through X-ray scans for years without comment, and it's handy for putting the screw back into the earpiece when it falls off my glasses (did it yesterday, in fact). My pocket knife is in my checked baggage. I also carry pen and paper for keeping track of things that might come in handy, things we forgot and should have on a list, good restaurants, phone numbers, and such.

I'm sure I'm missing the real question. How is it that you propose to use the compass?

As others have said, my wife and I carry band-aids, OTC pain- and gas-killers, books and water. We also have flashlights and whistles, 2-way radios, books, water, and the like. She carries a blindfold because she doesn't sleep well in lighted areas.

We both also carry a couple of days' worth of clothes in our carry on, along with any medicine we need, cameras, passports, hotel & car-rental confirmations, tickets for whatever we have tickets for - basically everything we need assuming our checked bags won't arrive. (I wear a photographer's vest which is essentially another carry on bag disguised as clothing.)

You're going to Britain, so this won't apply; we fly to the Caribbean, and I always carry 2 or 3 folding fans (the hand-powered kind you fan yourself with) because we board planes that have been standing in the tropical sun and sit there while they finish loading the luggage (for other passengers, of course - the people on earlier flights - our luggage is loaded onto later planes :->) with propeller engines off for the safety of the ground crew.

As with others, our major survival issues have been (a) laying around the airport because the plane didn't arrive and won't and the island has no other means of escape or (b) none of our checked bags made it to our destination and the airline is telling us "your luggage will be on the next flight down," but the flights are WEEKLY, "well, that won't be a problem, will it?" (I can think of a use or two for the compass in those situations, but they're not exactly survival.)