I use the 24 hour clock on everything, and have done since I was about 10 years old. Mainly because thats when I got my first mobile phone, and started sending/receiving text messages, and there's nothing more annoying than receiving a message saying "ill meet u 2moro @ 10" and having to ask for clarification.
The metric system is odd here in the UK. All products have to be marked with metric sizes/weights (except draft beers which are sold by the pint), but our road signs, speed limits and speedometers are marked in miles rather than kilometers... but your car doesn't have a nought to 60 time, it's nought to 62 (100Kmph). Celsius or Fahrenheit depends on whether it's hot or cold, and you use the one which sounds the most dramatic.
I mostly stick to metric measurements (because it's what we're taught in school, they're usually more precise, and I spend a lot of time in Europe where everything's metric), but still use inches, feet, yards when a) that's the correct measurement b) I'm talking to someone over 40.