In addition a police caution includes an admission of guilt and it gives a person cautioned a criminal record. That could have had serious consequences for JM in relation to her teaching prospects and her ability to emigrate to Canada. She was in fact let off scot free from several serious criminal offences, leaving aside the possibility of prosecution for perverting the course of justice or assisting an offender. Coupled with the lies about the NOW deal the jury were seriously misled and had they known the truth it is likely that they would have viewed her evidence at trial with far greater circumspection.
Yes, and if I may add a further point that occurs to me on reading your post. This is something I think you are already implying, but it needs to be spelled out.
If I was of a cynical frame of mind, I might well conclude the following:
(i). To borrow from criminal parlance, the caution for drug offences could be regarded as the prosecution's 'convincer' in anticipation of the jury's concerns that this was just a 'dirty deal'.
(ii). The police and the DPP may have decided to caution here for the drug offences rather than the dishonesty offences so as to play down the dishonesty aspect of the criminal history of a key prosecution witness whose evidence contains no corroborating facts whatsoever and thus whose entire contribution to the case against Jeremy rests on her own say-so.
(iii). The police and the DPP may have issued the caution and other disposals listed above at or just before the point when questions were being asked about this witness at the trial.
Incidentally, regarding her admission to Canada, the caution will not have affected her much. It has long been settled law in Canada that, at least as far as UK citizens are concerned, Canadian immigration law aligns with the UK's 1974 Rehabilitation of Offenders Act. This means that, although she would still have had to declare the drug caution, is it quite likely the Canadian authorities simply disregarded it, and would have done the same for any other disposals and incidents such as arrests.
The name of the relevant Canadian case escapes me for the moment, but even if it was not current at the time of Jeremy's trial, I suspect Julie Mugford's treatment by Canadian immigration would have been something like what I have just outlined, especially given that her entry to Canada was against the background of marrying a Canadian citizen. In other words, I doubt there was some sort of 'back stairs' arrangement between the authorities of two friendly countries to re-settle this obscure person.