Sedwill also said that Russian Federation has tested means of delivering chemical agents "including by application to door handles", pointing out that the highest concentration of the chemical found after the attack was on Skripal's front door handle.
The experts did not publicly confirm it was Novichok but said it was the same as that claimed by the United Kingdom, and that it had an "almost complete absence" of impurities.
The UK continues to deny Russian Federation information about the whereabouts of Sergey Skripal's daughter and her state of health.
It also received parts of samples taken by the British authorities which were analysed in OPCW-designated laboratories for "comparative purposes".
"There can be no doubt what was used and there remains no alternative explanation about who was responsible".
According to the embassy's maycommentary, "the case against Russian Federation is based on three elements: the identification of the chemical agent, far-fetched speculations and conclusions regarding the "operational experience" and "motive", as well as unverifiable 'intelligence'".
"This is based on testing in four independent, highly reputable laboratories around the world".
"There can be no doubt what was used and there remains no alternative explanation about who was responsible - only Russian Federation has the means, motive and record", he said. "The Kremlin must give answers".
The poisoning led to President Donald Trump expelling 60 diplomats from the U.S. All told, some 18 countries have expelled Russian officials.
In retaliation, Russian Federation also expelled 60 American diplomats.
Skripal, 66, and his daughter, 33, were admitted to a hospital after being found unconscious on a public bench on March 4 in Salisbury.
She also thanked Viktoria Skripal for her concern but asked her cousin not to "visit me or try to contact me for the time being".
Russia's ambassador to the, Alexander Yakovenko, dismissed the charges Friday as unfounded and untrue.
Viktoria had been quoted widely in the Russian media following a phone conversation she claimed to have had with Yulia from hospital last week.
"With no possibility to verify it, the publication by the Metropolitan Police raises new questions rather than gives answers", the embassy said.
"We can not be sure that Yulia's refusal to see us is genuine". 'So far, we doubt it much.
"The clinical picture", he added, showed that the nerve agent BZ had been used on the Skripals.
In the statement published on Wednesday, she said her father remained seriously ill and that she was still suffering from the effects of nerve agent used against them.