The UK government is aware of plans (or possibly itself planning) to assassinate Wikileaks' Julian Assange. That's the only conclusion which can be drawn from their response to a Freedom of Information Act request about the subject:
The FCO can neither confirm nor deny whether it holds any information that would meet the terms of your request, in reliance on the exemption contained in section 27(4). Section 27 relates to international relations. Section 27(4) is a qualified exemption under which we have no obligation to confirm or deny whether information is held where to do so would or would be likely to
(a) prejudice relations between the United Kingdom and any other State or
(b) prejudice the interests of the United Kingdom abroad.
But given the explosive nature of the request (for any and all records held by FCO "which concern current or previous plots to assassinate Julian Assange, including those generated by UK and US security agencies such as MI5, MI6 and the CIA"), I'd expect them to deny it if they possibly could. The fact that they haven't suggests strongly that they are aware of at least one such plot. Whether they're withholding it to protect the reputations of their murderous "friends" (who would be upset if the UK spilled the beans on their past or present murder plans), or to protect their own reputation is unclear, but either way: they're actively covering up for would-be murderers.