Final answer:
The Lord High Chancellor is the official who used to be the Speaker of the House of Lords and now leads the UK's Justice ministry.
Step-by-step explanation:
The official who once served as the Speaker of the House of Lords but now heads the Justice ministry in the United Kingdom is known as the Lord High Chancellor. This position is closely associated with the functioning of the country's judicial system and is separate from other executive roles such as the Chancellor of the Exchequer, Prime Minister, and Secretary of State. The duties of the Lord High Chancellor have evolved over time, and while this official no longer presides over the House of Lords, they still play a significant role within the UK government.