@Anton: coming at this one completely cold
I know next to nothing about the circumstances surrounding Yuri's death, and you may well know things I don't, but I need much better evidence than what you have stated above to make a conspiracy rather than a cock up theory fully credible. Cock ups are very common, especially amongst testosterone filled test pilots who are known not to be particularly risk averse by profession. His visit to Manchester a few months after his spaceflight was so well received in the UK that Gagarin must have been seen as a potentially useful ambassador for the Soviet regime in any case:
http://www.wcml.org.uk/contents/international/cold-war/yuri-gagarin-in-manchester/
Conspiratorial eliminations of inconvenient and popular figures seem likely, by their very nature, to leave evidence of the kind surrounding the Kennedy assassination (plenty of smoke and we have a saying "no smoke without fire", but still unproven):
a. Important suspects in the plot themselves being eliminated soon afterwards to prevent them from talking.
b. Other bits of evidence of a plot (e.g. witnesses hearing gunshots and seeing smoke somewhere [grassy knoll] not accepted as relevant within the official explanation.
c. Elaborate official explanations of gunshot trajectories stretching credibility, requiring bullets to have to have ricochetted in order to have penetrated the victim more than once.
Without better evidence than you have stated above, Occam's Razor tends to point to cockup (e.g. to normal human laziness in signing safety check boxes without people really checking) as opposed to conspiracy as being the simpler and more likely explanation.
Is there any evidence of dissenting views having been expressed by Gagarin ? If not then we are lacking a crucial factor to make any conspiracy theory credible: i.e. the need for a motive, for which in the case of Kennedy, as a charismatic, liberal and reforming US president, he was making enough enemies.