Westminster Abbey was amazing. Unfortunately we weren't allowed to take photos inside but it was spectacular. The current building started being built in 1245 for Henry III. There is so much Royal history in this place. Some that stand out for me are the tombs for Henry III, Henry VII, Anne of Cleves (Henry VIII wife), Mary I, Elizabeth I, Mary Queen of Scots as well as many other monarchs and notable people.
We also walked past the Palace of Westminster which houses the British Parliament - House of Commons and House of Lords. It's also where Big Ben is :D