UK voters do have the power to replace the House of Commons at a general election: UK Parliament says all constituencies become vacant and a new MP is elected for each, and every sitting MP loses their seat at the end of Parliament. (parliament.uk) But the phrase "sack everybody" is too broad: Parliament says you can only vote for your local MP, not for a new Prime Minister, and the Prime Minister appoints ministers. (parliament.uk) So the claim is broadly right about replacing elected MPs and changing the government, but false if taken literally to mean all officeholders. Sources: UK Parliament, "How MPs are elected"; UK Parliament, "How does an MP leave office?"; UK Parliament, "General elections".