Good point. Although it was a good few days before she was found and drugs can not show a trace, presumably, after a period of time. Unless that process is ceased once she has died as most of those days she was already dead. I think it was well planned - as said at court, he had been doing regular trips to London for about a month to prepare. He knew the roads well, travelled them regularly, he'd know he'd have a straight run at that time of night. But maybe all part of the thrill - the fear of being stopped. If stopped he'd just flash his police card presumably. But probably confident he wouldn't be.