Electronic devices security on airplanes

There are basically four places your possessions can be during the flight:

• in a bag in the overhead compartment. In theory, someone could keep track of which bags belong to which passenger, and when you're asleep or away from your seat, pull down your bag, remove an item, put it in their bag and replace your bag before you return. This is insanely risky for them and the chances of being caught (by you or other passengers) are high
• in the seat pocket. Your seatmate or others nearby will notice if anyone else removes something from this. The risk for you here is leaving the device in the pocket when you deplane. I know dozens of people who have done so and none who have had something stolen from the overhead
• in a bag under the seat in front of you (assuming there is one) - this makes it relatively easy to get things in and out, but interferes with your foot space.
• in your hands or pockets. The safest, but not practical when you're headed for the bathroom.

My approach is to use a small cloth "shopping bag" - the kind that comes in a tiny pouch. When I get on the plane I put all the stuff I will actually use during the flight - laptop, magazines, gum, phone, headset, bottle of water, whatever - into this bag and put all my carryon away. (I started doing this when I got seats with no "under the seat in front of you" available.) I can usually keep this bag with me during takeoff if it has very little in it and is beside me on the seat or on my lap. Some airlines have "purse hooks" I can use to hang it from. You might not recognize them, so here is an action shot:

Other times I put this bag into the seat pocket - I can't possibly forget it, it is a distinctive colour and pattern. This leaves my pockets empty, saves me from having to get up to get things in and out of my carryon, and I suppose also adds a little security if I were worried about that sort of thing.

I have travelled quite a lot on both cheap and expensive flights and have never witnessed or heard about theft on the flights, even the cheapest of low-cost flights, although that doesn't mean it doesn't happen.

I have, however, sometimes wondered the same thing when travelling long-haul to notorious destinations like Mexico/Colombia. Things I have considered in order to make my travel more secure are mainly around keeping what I care about close to me (eg. phones, tablets, laptops), either on my person (pockets) or beneath the seat in front of me.

I wouldn't consider a BA flight to America a risky flight. You'll be able to gauge the character of the people sitting around you pretty quickly and you'll more than likely be reassured that they would be as alarmed as you of someone rummaging around an overhead bin that didn't contain their luggage.

Ultimately though, there's always going to be a small risk, and if you can't afford to replace something you either have to keep it close or not take it at all.

A note about checked-in luggage. I never put a lock on my suitcases, since the TSA or equivalent can cut locks off or cut into bags if necessary, and a thief can easily get into a bag. I do put valuable items into my suitcase but never anything too significant (expensive/heavy chargers rather than an SLR/laptop). This is party because of the risk of baggage or item loss, but mostly because of the risk of damage from careless baggage handling.