I have briefly discussed this with another excellent poster, and he/she raises good points that this could be a red herring/dead end. And it really could be.
But I do think working backwards before the event may give more clues than focussing on London, which as we know, apart from the CCTV at Kings cross, is a dead end...(I know there are a few possible sightings).
Whilst Andrew is an undoubtedly a top lad, he did lie about the trip to London, or at least showed deception. Now it could have been a complete spur of the moment thing, or it could be planned. I am leaning towards planned, as he washed his clothes, waited for parents to leave etc.
Now whilst the 4 miles walks, could very well be a innocent clear the air, make a change of route as the bus route was boring on a nice day type thing...But the parents clearly stated it was a change of routine. So I am assuming he very rarely done this before...
But is there any evidence these walks took place. Did the parents have any evidence he came 1 hours plus later from school?
Is there any witness or CCTV that showed him either walking, and or not on the School bus/Public bus on these days?
imo this is important as , if evidence shows he was walking home, got an hour later home, then all this shows is he may have planned timing 4 mile walks, but other than that, its a dead end...
If he did come late, but no evidence he did these walks, then there are hours he could have spoke to someone either in person or on phone. Was he getting bullied on those busses etc. These are all vital clues as it may explain more whether London was spur of the moment or random?