Zelenskiy met King Charles at Windsor Castle ahead of political meetings later in the day.
The two shook hands for cameras on what was their third meeting this year and the latest gesture of Charles', and the United Kingdom's, support for Ukraine.
The Ukrainian president later met Starmer and the speakers of both houses of parliament.
"We will also be negotiating new and powerful steps to increase pressure on Russia for this war and to put an end to the strikes," Zelenskiy said on X following a deadly Russian attack on the Ukrainian capital.
Officials in Kyiv said nine people were killed and 33 injured in an overnight assault on the Ukrainian capital.
Mayor Vitali Klitschko said the casualties occurred during a strike on a residential building.
Ukrainian Interior Minister Ihor Klymenko described the attack as involving heavy rocket and drone strikes.
Zelenskiy's spokesman said the president would also talk to Ukrainian military personnel who are being trained in the UK and representatives of think tanks.
"The main purpose of the visit is to deepen defence co-operation," the spokesman said.
Ukraine's anti-corruption authorities said on Monday they suspected a sitting deputy prime minister of receiving a $US345,000 ($A534,500) kickback, the latest investigation into a high-level Ukrainian official over graft.
Ukraine has stepped up its efforts to crack down on corruption as it seeks membership in the European Union while also fending off Russian forces in the more than three-year-old war.
Officials did not name the minister, one of five deputy prime ministers, who they suspect of abusing power by approving the development of state-owned land in a scheme that would have led to a loss for the state of about $US24 million.
The alleged crime took place while the suspect was serving as a minister of regional development, according to the National Anti-Corruption Bureau of Ukraine (NABU).
The plan, which allegedly involved five other suspects, was halted when anti-graft officials obtained a court-ordered seizure of the land, NABU said.
The EU has made fighting corruption a top prerequisite for Ukraine to join the 27-member bloc.
The deputy minister is among the highest-ranking officials targeted by anti-corruption authorities.
Investigations have also focused on MPs and a former deputy head of the presidential administration.
with DPA
