What could Julie be charged with as far as being an accomplice goes ?
Did she supply the murder weapon for Bamber ? No.
Did she loosen the bathroom window for him ? No.
Did she say Bamber was on the phone to her between 10pm - 3am so could not have left gone to WHF ? No.
Did she draw up confidential plans of the inside of WHF ? No.
She could only be charged with knowing Bamber intended to committ the massacre. I assume. In that case she would just say she didn't believe him.
Or she could be charged with withholding evidence. For 20 days. That is negated by her approaching the police, giving a full WS & testifying.