Well, Statement in lieu of prospectus is issued by a public company having share capital that intends to raise capital privately that means not publicly, though the contents are almost similar of both prospectus and statement in lieu of prospectus but statement in lieu of prospectus is to be filed with the registrar by the public companies going for issue of capital privately.
Julia is right. The concept is that even though you wish to allot shares privately, some details of the company which may not be found in the Memorandum/Articles of the Company are needed by the Authorities (usually Registrar of Companies). These are then given through the Statement in lieu of prospectus. So, in short, you give prospectus when you invite the public to subscribe for any shares. You give a statement in lieu of prospectus if you do not wish to invite the public to subscribe for any shares, but wish to allot the shares privately.