I seem to remember earlier reports that he handed in his firearm around the time of SE's disappearance because he was stressed and didn't want the responsibility.
Updated - found various reports in MSM:
Sarah Everard’s body found in builder’s bag, court told
"On March 2 he began a 12-hour shift at 7pm before going on leave.
He was due back at work on March 8 but on March 5 he reported that he was suffering from stress.
On March 6 he emailed his supervisor to say he did not want to carry a firearm anymore."
Whether this stress was genuine as a result of what he'd done/fear of being caught or he was already thinking of his options, who knows - but his behaviour in custody suggests he wasn't calmly plotting his defence (banging his head etc)