You can write to members of the Royal family at the following addresses:
His Majesty The King
Buckingham Palace
London
SW1A 1AA
The Prince of Wales and The Duchess of Cornwall
Clarence House
London
SW1A 1BA
Her Royal Highness The Princess Royal
Buckingham Palace
London
SW1A 1AA
His Royal Highness The Duke of York
Buckingham Palace
London
SW1A 1AA
Their Royal Highnesses The Earl and Countess of Wessex
Bagshot Park
Bagshot
Surrey
GU19 5PL
Their Royal Highnesses The Duke and Duchess of Gloucester
Kensington Palace
London W8 4PU
His Royal Highness The Duke of Kent
St. James’s Palace
London
SW1A 1BQ
Her Royal Highness The Duchess of Kent
Wren House
Palace Green
London
W8 4PY