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I have yet to have my carry on bags checked at Heathrow or Gatwcik but have never flown with Lufthansa nor to the USA. If you are in the position to shop around you may find considerable variation in carry-on alowance. That said, the luggage reglations for flights to America, last time I looked, are relatively horrific.
I tend to put things like chargers, batteries, flash guns (wrapped in bubble wrap) in my suitcase.