I will have a layover in London, and plan on heading out early in the morning to see the city and grab breakfast. Where are the best places either near King's Cross or by Big Ben to grab food at like 7am?
St Pancras Railway Station (across the road from King's Cross station) has every coffee shop you have heard of as well as more expensive restaurant type establishments, which will be open.
If you want a real London experience, it could be worth heading over the Smithfields Market (nearest tube stop is Farringdon just 1 stop from Kings Cross http://content.tfl.gov.uk/standard-tube-map.pdf). It's the traditional meat market in London and operates overnight, so you'll see porters in the pubs having a pint first thing in the morning, as it's basically their dinner time! I really like breakfast in the ground floor cafe area at Smith's which opens at 7am https://www.smithsofsmithfield.co.uk/