No evidence is provided to support the claim.
No British base is being built in Guyana so far. On August 16, the Ministry of Foreign Affairs of Guyana strongly denied the allegation and called on Russia to withdraw it.
In late December 2018, Britain’s Secretary of State for Defence, Gavin Williamson, declared during an interview with the Sunday Telegraph that his Government was looking for two new permanent military bases in the Caribbean and South East Asia, as part of an effort to make the UK “a truly global player” after Brexit. The same newspaper speculated that the location for those bases could be either Singapore or Brunei in Asia, and either Guyana or Montserrat in the Caribbean. Almost immediately, the announcement was criticized by Russia’s Foreign Ministry. But there is nothing to signal that this project is already under construction, much less than Venezuelan refugees are being trained by British forces.