A Theory
This is, of course, just a theory and I have had many over the years, but this one doesn't seem to have come up.
So, Andrew heads to London for the day, and his intention is to do exactly as his Dad predicted. He is going to rock up at one of the houses of the people he knows in London, and stay over there. Emergency over, problem solved, parents just happy to know he is safe and well.
So this goes to plan initially, he does whatever he wanted to do and turns up at this persons house, family member etc. And then the theory goes in one of two directions.
Either something happens when they enter the home of this person, or, the person isn't in and Andrew is stuck outside and a neighbour of this trusted person invites them in out of the cold, and then something happens.
So, if I was the police I would be looking at the people Andrew knew in London, whether they had any lodgers etc at the time, and also their neighbours at the time and whether those neighbours have since have been convicted of anything since then.
The likelihood is still, even now, that if Andrew was murdered, he was murdered by someone he knew, it's why the police were so focussed on the Dad. Maybe the Police were right, but just looking in the wrong place.