“I wasn’t sure I’d make it back to Kyiv so coming back is an extraordinary thing,” Melinda Simmons said in an interview with the Observer.
“It absolutely feels like the right place to be,” she said, although she acknowledged the potential risks.
Russia has previously made threats against diplomats based in Kyiv.
Earlier this week, a Russian statement said forces were primed to carry out strikes against “decision-making centres in Kyiv”. It added that the presence of foreign advisers in those sites “will not necessarily be a problem to Russia deciding to take retaliatory action”.
Britain started withdrawing some embassy staff from Kyiv in January, shortly before the war started. Ms Simmons left Kyiv in late February, although stayed in Ukraine for a little while longer before returning to London.
But Boris Johnson announced last month that the British embassy in Kyiv would be reopening.
Ms Simmons told the newspaper that as she drove into the city on Friday she got “a real sense of what was going on… and it’s truly shocking”.
“But what is equally extraordinary is to see how Ukraine kept Russia out of Kyiv. Every way in which Ukraine has been able to do that is a thing to celebrate and to treasure,” she said.
Russian forces withdrew from around Kyiv after failing to seize the city, and is now instead focusing its assault on the eastern Donbas region.
Ms Simmons said she expected the war would likely last “certainly through this year and probably through next year”, which chimes with other warnings from the West that they must be prepared for the long haul.
And she made clear she was not minimising the risk facing her, adding: “I am here with a heightened level of security protection and bearing in mind that potential risk, but for now I feel comfortable working under those circumstances.”
Although the PM said the embassy was reopening soon, Ms Simmons said the Kyiv embassy had not yet reopened and they were not running a consular service.
She also repeated the British government’s advice that UK citizens should not travel to Ukraine. Earlier this week, the Foreign Office announced one British national was killed and another was missing.
Meanwhile, Mr Johnson spoke to Ukrainian President Volodymyr Zelensky over the phone on Saturday, and was given an update on the fighting in eastern Ukraine, as well as the situation in the southern port city of Mariupol.
Mr Johnson confirmed the UK would continue to provide additional military aid for Ukraine to defend themselves, Downing Street said.
“The leaders also discussed progress of the UN-led effort to evacuate Mariupol and concern for the injured there. The prime minister offered the UK’s continued economic and humanitarian support,” a Downing Street spokesperson added.