WK007 -- The Scout Association say
Scout Association factsheet "Fundamentals of Scouting" (here )
"it is not possible for an atheist to hold a Warrant as a Leader"
Scout Association Equal Opportunities [sic] Policy (here )
"The Scout Movement is not a religious organisation, but does expect its Members to follow a religious faith and conform to its practices. A belief in the spiritual aspect of human beings and a belief in a spiritual power greater than ourselves is fundamental to Membership of the Scout Movement [...] While it is not acceptable to deny a volunteer entry to the Movement because of their membership of a faith community, a person would not be suitable as a Leader if he or she were an atheist"
Scout Association Noticeboard article "Promoting Multi-Faith Scouting" (here )
"We are aware that in some newspapers an incorrect statement is made that Scouting is open to atheists. This of course is not true and we have gone back to those newspapers to clarify this point."
none of which suggests to me that if you have no God at all it's up to you. It suggests to me, fairly clearly, that Scouting isn't open to atheists. Except, if you are splitting hairs, to atheists who pretend not to be atheists. There have been genuine cases of leaders being busted out of the movement for being atheists.
The Guides don't have the equivalent documents accessible on their website, but I am tolerably certain that their policy is the same (because every time a journalist asks, they say it is). By all means ask your district staff what the position would be regarding a leader who openly admits to being an atheist -- I will be pleasantly surprised if it wouldn't be a problem.