INSIGHT-How Boris Johnson helped Ukraine with its weapons shopping

* On weekly calls with Zelenskiy, Johnson collected "shopping list" of arms

* UK has helped governments needing political cover or logistical help

* As Ukraine's supplies tighten, political choice looms: double down or push for talks

* With cost-of-living crisis at home, public support for arms may fade

By Elizabeth Piper, Andrew MacAskill and Tsvetelia Tsolova

LONDON, July 8 (Reuters) - Ninety minutes after resigning as British prime minister on Thursday, Boris Johnson called Ukrainian President Volodymyr Zelenskiy. He told the Ukrainian leader his people had the UK's unwavering support in its fight against Russia, and said Britain would continue to supply vital defensive aid for as long as needed. "You're a hero, Volodymyr," he said, according to an aide who listened to the call. "In this country, everybody loves you."

In the months since Russia invaded Ukraine, Britain has become an important go-between for Zelenskiy, officials in Britain and the United States told Reuters. Part broker, part delivery service, supporting Ukraine has been a crucial part of Johnson's premiership. Ukrainian Foreign Minister Dmytro Kuleba described him as a "true friend of Ukraine" in comments sent to Reuters by his ministry.

Whoever runs Britain in coming months will face important decisions about how to pursue the policy. Johnson told Zelenskiy he still had "a few weeks" to keep the support going, according to the aide. But Britain's leadership will be at a point of transition as Russia is steadily gaining ground in what Moscow calls a "special operation."

Until that last call, Johnson had spoken to Zelenskiy 21 times since the start of the war – once every six days on average. Their conversations often opened with Zelenskiy reading out a "shopping list" of arms, three UK officials with knowledge of the matter told Reuters. The calls could "be very transactional" at the start, one of the officials said.

As an example of the deals that followed, London and Oslo agreed that Britain would send multiple launch rocket systems (MLRS) to Ukraine and receive similar, older equipment from Norway in return that it can modernise, Norway's government confirmed. In May, Ukrainian defence minister Oleksiy Reznikov thanked Britain for its help delivering Denmark's anti-ship missiles.

Britain also recently made an inquiry for 27 Soviet-designed heavy machine guns from Bulgaria, according to a document from state-owned arms company TEREM seen by Reuters. TEREM told Reuters that that deal did not go through.