The BBC wesbite has a section about Orwell and whilst it has some information about his decision to leave, specifically his resignation letter. I think both the letter and the site however are missing out the major points in his decision to leave.
What Orwell saw at the BBC during war time helped him to create the Ministry of Truth after all.
Anyway, I think some of the more important points about is decision to leave are included in this little audio clip, reproduced with kind permission from the author Michael Shelden whose Orwell book is probably the best biography of him that is out there.
For more on the BBC you could try listening to this.