While I can't speak for UK law, the hotel would almost certainly be liable for the theft in the US, even if they handed the owner a claim check that stated they weren't liable for it.

The problem is that once the passenger hands over their luggage to the airline, they no longer have the ability to protect it (and let's face it, luggage locks provide such little security as to be effectively meaningless). It becomes the airline's duty to protect and care for that luggate as established both in common law and in the Montreal convention (up to the prescribed limits). The convention isn't really there to protect to passengers. It really exists to limit the liability of airlines, who otherwise might be held liable without limit.

Yes, the OP was foolish to have put an iPad in checked luggage. The OP is paying for that foolishness by having to waste lots of valuable time trying to get compensation. That doesn't release BA from its liability, though.