You rarely see flags flying in the UK - it's nowhere near as common as in the USA except around the palaces. This is at the end of the Queen's driveway to Buckingham Palace, just by Trafalgar Square. It's the flag of St George - the patron saint of England and the Union Flag in the corner. There is a lot of history behind the way the flags are designed and I'm not sure if this has some other meaning than it's an England flag as part of the United Kingdom.