We do not need to prove anything because the UK grossly violates Article 36 of the 1965 Convention signed by the USSR – it is functioning now – and Great Britain according to which a consular officer has the right to unimpeded contacts with a citizen of his country in the situation when something happens to him.
Russian citizen Yulia Skripal has said that she did not wish to take up the offer of assistance from the Russian embassy in London. news.met.police.uk/news/statement-issued-on-behalf-of-yulia-skripal-302508
Many competing and contradictory stories promulgated by Russian state-controlled media on the Skripal poisoning https://euvsdisinfo.eu/disinformation-cases/?text=sergei+skripal&disinfo_issue=&date=
On Sunday 4 March Sergei and Yulia Skripal were poisoned in Salisbury with a military-grade nerve agent of a type developed by Russia.
For the UK response see: https://www.gov.uk/government/news/novichok-nerve-agent-use-in-salisbury-uk-government-response
The OPCW have confirmed the UK findings in the case. "The results of the analysis by the OPCW designated laboratories of environmental and biomedical samples collected by the OPCW team confirm the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury and severely injured three people." https://www.opcw.org/news/article/opcw-issues-report-on-technical-assistance-requested-by-the-united-kingdom/