It is the Houses of Parliament for both House of Lords and House of Commons. This UNESCO World Heritage was built in Middle Age and damaged or demolished many times by fire accidents or war.
The existing building was completed in 1840. However, a huge part of Palace of Westminster was destroyed by bombs of Germany Air Force during World War II.
It was rebuilt in 1950. The tower at the left side of photo is Victoria Tower. The flag of Royal Standard will be rose when The Queen in there.