If you plan to murder someone entirely on your own then, as long as you don't actually carry out the act (or try to), you can't be convicted of an offence. i.e. if the police find that I've got detailed plans of my enemy's movements, together with notes on the best way to kill him and the relevant weapons in my possession then, apart possibly from the possession of those weapons, I'm not guilty of any offence. 'Planning to commit murder' isn't a crime. (The only exception is when that murder is part of an act of terrorism; 'planning to commit an act of terrorism' IS a criminal offence).
However '
conspiracy to commit murder' IS a criminal offence. ('Conspiracy' has to involve two or more people; you can't 'conspire' with yourself). The fact that no actual attempt to commit the murder has taken place is irrelevant; if it can be shown that there was serious intent to murder (with the plan involving two or more people) anyone involved in the plan can be charged under Section 1(1) of the Criminal Law Act 1977. Under Section 3(2)(a) of that act, "the person convicted shall be liable to imprisonment for life":
http://www.legislation.gov.uk/ukpga/1977/45