Flask for tea/coffee is essential, especially at this time of year. For a high energy sandwich i would use egg mayo, corned beef and spinach. Other options would be a cornish pasty, or some kind of cold pasta dish (which you can make up in advance, a few days worth), or some homemade soup of some kind in a flask. Banana is the obvious fruit option, and for something sweet, a flapjack or similar. Oh, and as has been mentioned, a good breakfast is the best way to start the day!.