We do NOT support an entire Royal Family. We support the monarch and their spouse, because the spouse is an integral part of the liason with other heads of state which a head of state performs. We supported their dependent children, of course, but the Queen's children are now all grown up.
"The Sovereign Grant Act 2011 came into effect on 1 April 2012. It sets the single grant supporting the monarch’s official business, enabling The Queen to discharge her duties as Head of State. It meets the central staff costs and running expenses of Her Majesty’s official household – including official receptions, investitures and garden parties. It also covers maintenance of the Royal Palaces in England and the cost of travel to carry out royal engagements such as opening buildings and other royal visits.
In exchange for this public support, The Queen surrenders the revenue from The Crown Estate to the government which for 2018-19 was £343.5 million. The Sovereign Grant for 2020-21 is £85.9 million which is 25% of £343.5 million. Section 2 explains how this is calculated.
In accordance with the Civil List Act 1952, HRH Prince Philip, Duke of Edinburgh receives a Parliamentary annuity of £359,000. Although, the Duke of Edinburgh has stepped back from public official duties, he still requires office support for non-public official duties. The Duke of Edinburgh is Patron, President or a member of over 780 organisations, with which he continues to be associated, although he no longer attends engagements.
The Sovereign Grant used to be the Civil List, which was set up in exchange for the income from the Crown estate, which itself was, centuries ago, once the private property of the monarch, from which the king or queen paid the officers who assisted them in running the country. The Crown Estate is now held in trust for the nation, which keeps 75% of the income, giving 25% to the monarch as the Sovereign Grant. So the country gets three times as much from the Crown property as the the queen does, and from her 25% she pays lots of salaries and does lots of entertaining and admin and so on. The Sovereign Grant business accounts are scrutinised by Parliament.