The visit was delayed twice, once while Spanish politicians formed a new government and again last month because Britain held a snap general election.
The Spanish royals will be greeted on Wednesday by Queen Elizabeth II -- a distant cousin of Felipe -- with a ceremony in central London.
Ana Romero, author and former royal correspondent for Spain's El Mundo newspaper, said the visit is the "jewel of the crown" of the king's calendar.
"The pomp has its importance because it is the moment which the monarchy has to demonstrate its diplomatic usefulness," she told AFP.
Gibraltar on the menu?
Felipe is due to address the British parliament, where he could follow in his father's footsteps and talk about Gibraltar -- although the political landscape has somewhat changed since 1986.
At the time, King Juan Carlos said the sovereignty of the British territory was "the only thing that separates us".
The EU has already promised Spain a veto over the extension to Gibraltar of any future trade deal between Britain and the bloc, a topic which could come up during the king's lunch on Thursday with British Prime Minister Theresa May.
With a population of just over 32,000, Gibraltar has been a British overseas territory since 1713 but Spain has long laid claim to the rocky outcrop.
The fate of an estimated 300,000 British citizens living in Spain -- the majority of them retirees -- may also be up for discussion along with that of around 116,000 Spaniards living in Britain.
"Most importantly, we must give priority to our citizens, be it the British here, or the Spaniards there," Britain's ambassador to Spain, Simon Manley, told Spanish public radio RNE.
"We are proud of the contribution from the Spanish, be it nurses, engineers, and we want them to stay," he said.
Romero suggested that while such topics will likely come up during the visit, the king also has other priorities.
"It would be logical that he will allude to the sovereignty dispute over Gibraltar, as his father did, as well as to Brexit and jihadism, since in the most recent attacks in London a Spaniard died as he tried to defend a woman," she said, referring to an attack on London Bridge on June 3rd and the death of skateboarding hero Ignacio Echeverria..
Business will also be on the agenda and top Spanish business leaders will accompany the royals, including from Ferrovial, a Heathrow airport shareholder, Santander bank and telecoms firm Telefonica.