She would have to be as absolutely cold as ice to do that and then just carry on as normal until her and Jeremy split.
I think it's far more likely that her story was simply cobbled together, as a result of external pressures, a build up of mumerings and the realisation that Jeremy was going to take a fall.
I tend to agree with you Roch, if we take the view that Julie didn't in any way suspect that JB was involved at the time of her identifying the boys, staying at Colin's, accompanying him to the funeral and various getaways we could believe that by the time she went to the police she may have been troubled by insinuation and murmerings as you suggest. Adding to this her memories of comments and threats by Jeremy about his family over the time she had known him may have caused some kind of realisation to take root in her mind until it became too strong and she couldn't ignore it any more therefore she shared it with friends and rightly they persuaded her to go and speak to the police.
She would have been a gift to the police who could easily have worked on her convincing her and reinforcing her fears that he had done it, and explaining that they needed her to be a witness for them to ensure he was locked up for good. Her past criminal behaviour gave them an added lever to 'persuade' her of his guilt and her need to point the finger at him.
If JM knew JB was responsible from the start as is the official claim she would have to have been a young woman without any moral compass at all or as you say 'as cold as ice' to behave the way she did and although her behaviour before the crime was very questionable since the trial she does seem to have grown up, had a good career and lead a decent life without any sign of criminality etc.
It's good to look at things from different angles sometimes as there are many possibilities and as far as we know any one of them or none of them may be the truth.