It’s available to pre-order from the 3rd of June, released on the 17th.
The core rules for the game will be free and available online. There is a separate, optional book for £30 that covers the narrative progression and provides core rules as well as advanced rules (things like Cities of Death, Stronghold Assault, etc. Stuff that used to be released as supplements).
The book is also included in the new starter set for £95.
You do NOT need the rulebook or the starter set to play 8th Edition.
You do, however need the updated rules for your army. There are five new books. They’re £15 each. Each book contains the rules for a number of armies. So simply pick up the book that contains your army and you’re all set.
The books are;
Index: Imperium 1; Space Marines, Blood Angels, Dark Angels, Space Wolves, Grey Knights, Deathwatch, Legion of the Damned
Index: Imperium 2; Imperial Guard, Adeptus Mechanicus, Imperial Knights, Imperial Agents (including the Sisters of Battle) and Talons of the Emperor (Custodes, Sisters of Silence, etc).