When traveling, it is always important to travel light. As I usually travel in the fall or early winter I need warmish clothes. Therefore, I travel in a skirt, blouse, sweater and have a warm winter coat, hat scarf and gloves as well as the usual underwear. One suitcase (one of those on wheels that turn every which way)and one carry on bag is enough. In my suitcase, along with toiletries, I will pack underwear, another skirt and 4 or 5 blouses, plus an extra pair of shoes. As the dollar is very weak against the pound, it is better to take your own shampoo, soap, toothpaste, etc. with you, so you don't have to buy them over there, thus wasting your spending money when you can be buying lots of really "useful" souvenirs. Best places to find the cheapest "flag-infested" items are around Trafalgar Square and along Oxford Street.
Of course, camera is essential as there are wonderful photo opportunities.