Large bags, rucksacks and suitcases are not permitted onboard – maximum bag is 18? x 13? x 8? or 46cm x 33cm x 20cm. Baby buggies that do not fold down are not permitted onboard.
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It's best to play it by ear and go on a day (or evening) when you are there. Day or night is chalk and cheese.........you can see everything clearly during daylight but not at night, however, London at night is twinkly and lovely. If this is your first visit go up in the day light hours.
You don't say what airline you're flying, but most US airlines allow one small suitcase and one other personal item, which is usually taken to mean a much smaller item, such as a purse or a laptop. If your backpack is a big travelpack, you probably will have to check one or the other.