My thoughts too, not even that person had a head start but that it is nearing almost a month after the fact now and person responsible for double murder is still not caught. Quite worrying on so many levels and also that this might turn into cold case.
With blood trail leading to certain exit, with at least one or more CCTVs on the street and neighbourhood and APR car recognitions cams in the area, multiple police assigned to this case, yes very worrying not seeing any results yet.
Another thought I entertained few weeks ago - if suspect was seriously injured he might have indeed seek medical attention anywhere else in London in following few hours or even days and not raising suspicion if claiming that he was himself attacked. It would not raise police eyebrows in other area as stabbings are so often in London.
There was middle aged man who died in other part of London next day after this murder due to stab wounds. (Sorry, don't have link as it was 3 weeks ago when I was checking). I know on the first look you would never connect those two incidents but think about it... it is very possible. Any such cases in following few days after the murder should definitely be checked out. Any of those injured/stabbed man anywhere in London could be the right suspect in double murder of the sisters.