It's obviously not irrational to think even democratic governments are capable of reprehensible actions, including against their own citizens (e.g.
COINTELPRO). It is irrational though to think they act against or take massive risks concerning their own interests for relatively minor payoffs. The damage to the UK government and specifically the Tory Party, both collectively and individually, nationally and internationally, if found to be involved in murdering someone on its own soil using a banned nerve agent would be huge and lasting. And the idea that May would not only want to want to take the risk of destroying her own party and position and doing enormous damage to Britain's reputation globally, but also of personally spending the rest of her life in prison, just so she could discredit Russia at a time when she's not even facing a leadership challenge and has three years to prepare to recover her standing for the next election (not to mention when Russia is constantly discrediting itself without her help) is barely worth commenting on. Even in the most desperate of plausible circumstances, the likelihood of the UK government getting involved in something this risky is close to zero.
Russia, on the other hand, and Putin in particular, have a known history of imprisoning and killing opponents at home and abroad, and have suffered little in the way of consequences for it. Putin is already considered to be a brutal dictator in the West and has almost absolute control of the media at home. Plus, without democratic checks and balances, the likelihood of finding proof he was personally involved is far lower. The relative payoff-to-risk ration for him then is considerably higher than that for May (although not so high that it couldn't backfire).
So, I don't think this is about whether we have an open mind* or not, it's more about basic political analysis. Even presuming all governments, democratic or otherwise, are capable of extremely immoral acts (which I personally don't think is an unfair presumption even if it is very unlikely to apply to all
individuals in those governments), most governments, particularly those in advanced democracies, must also be presumed to have enough collective intelligence to avoid not only metaphorically shooting themselves in the foot, but in cases like this, sitting on live grenades. And again, I emphasize the collective element; while Putin can essentially do what he wants with little consultation and likely has direct lines to the kinds of people or groups who could organize this crime, May, in order to carry it out, would have to go through processes at multiple levels of government involving very intelligent political players. The idea that they would all suffer the kind of simultaneous, calamitous lapse in judgment required to allow the operation to progress, and then suddenly recover the almost superhuman abilities necessary to cover it all up just isn't credible.
Pretty much the same principles apply to 9/11. Trump can't even manage to hush up a minor affair with a porn actress; Clinton, though apparently much smarter, got caught fiddling with an intern; Reagan, Iran-contra etc. Of course, all of these were orders of magnitude less damaging than being caught organizing 9/11 would have been, which explains why the risk was taken in those cases. So, bad judgment and venality, often in concert, isn't rare in government, but there are limits. Conspiracy theories continue to thrive though on the basic principle that governments are nasty and bad, while failing to adequately take account of the collective and individual consequences of the context in which politics is cultivated, i.e. power and self-interest. Once those pieces are added to the puzzle, the resulting picture, though often more boring, at least makes sense.
*(A Richard Dawkins' quote comes to mind— I paraphrase—it's good to have an open mind as long as it's not so open your brain falls out).