I think he is even more crafty than we give him credit for. He may have deliberately left some money untouched so that he could say (if ever the finger was pointed at him) 'look, I didn't even use the money'. Similar perhaps to how he tried to use the inheritance tax insurance - look I would have waited until after the wedding.
I really think he has huge issues around money. That sounds daft given what we know about his financial motive for the murder, but I think there is another element to it, from what I've seen of him. He seems to have a need to not only acquire money but to deny that it is important to him. And that's not just seen from the perspective of him denying he had a motive to murder, it's about him in general.
That's why I said a few days ago that I think when he was young he may have had to deny wanting something for himself. Possibly he suffered a narcissistic injury. [note here I am not blaming his mum! he had a dad too, and most probably a personality disorder.]
This is what Wiki has to say about narcissistic rage and injury -
I noted already his response to being accused of killing Helen for her money. The immediate snarl, when he was to all intents ignoring the officer's questions. He couldn't avoid responding, even if it was a non-verbal response. An example of body language letting him down.
I don't think his entire evidence was necessarily about defending himself against the charges. He boasted that he went to the jewellers with Helen with a budget of £20k in mind for the ring. And hid that he had asked Helen to embarrass herself and ask for a discount on the £14k price tag. All meant to make people believe he doesn't need money.
It's his Achilles heel, and probably not only to do with money, but intrinsic inadequacy.