It is strange behaviour not wanting a return ticket for all of 50p more. That wasn't the first time Andrew had been on the train (although perhaps solo) but certain he'd have known the difference between single and return ticket. He could even have got an off peak month return to cover him up to mid October.
My only query to that theory though is would he really have got back from a lift to Donny for the family dinner at 7pm? If the M1 is playing up it could take four hours to get back to Doncaster especially if the vehicle was parked in central London. Pretty much leaves him a couple of hours in London around lunchtime, is it really worth going down for that? Bit of an expensive Pizza Hut visit anyway!
Or was he happy rocking up at say 9-10pm even though his parents would've probably be worrying at that stage?
My other slightly long shot theory is he planned to go to event in London and then visit his relatives in Kent. His parents were encouraging him to go visit them that summer after all so he'd probably needed 40-50 quid to get a taxi there from central London?
However he got in unmarked vehicle and/or someone promising him lift and then awful things happened. That's why Worboys isn't someone you can 100% rule out given he was still at large in London in September 2007. If not him you only have to look at the Sarah Everard murder. London resident about ten minutes from her house just taken off the street and her life brutally ended.
That's where you'd fear for Andrew. Getting dark at around 7pm and him perhaps wandering quieter and edgy areas of London. He'd look very vulnerable to a motorist passing with malicious intentions.
IMO, the lack of a return ticket is one of those elements in this case that's considered to be factual, but it can't actually be confirmed.
There's no CCTV record of him in the Doncaster station, no way to time stamp this transaction, it's all up to the recollection of the ticket seller, and her statements had simply been paraphrased by Andrew's parents over the years. The claims that he "insisted" in not getting the return ticket, as if the seller had tried to explain the deal to him, for instance, are doubtful.
Some things to consider: Andrew had never bought a ticket to London by himself, and never (as far as we know) had went there with his family by train to return that same day. If he just asks for a ticket to London - unlike experienced travelers who already get to the cashier and ask for a one-way ticket or a two-way ticket -, the seller could just have asked "just a one way ticket?", and he said "yes". We don't know if this woman said something like "Are you sure? It will only cost you X more". Or if Andrew knew he wouldn't have to decide on a specific return time if he took the deal. (He could be refusing because he wasn't sure how much time he would spend there that afternoon.)
I don't give credit to the Pizza Hit sighting. There's no indication this was Andrew. For all we know he hopped into a car shortly after leaving King's Cross. And I don't see him relying on some to take him back to Doncaster. I believed he always planned to return by train. I do believe, though, he planned to meet someone he knew personally (I can't see him mustering the courage to go meet a stranger in the chat room - if not for concern worries, for his social skills.)
And he could have been planning to meet someone from Doncaster who was now living in London or someone he met somewhere else and who would be in London that day (big cities like this can become ideal meet up spots). And he might not have even been 'groomed' by someone who intended to kill him.
A likely possibility, to me, in this scenario: someone he knew from church; an older guy who was now living and studying in London and with who Andrew reconnected weeks before (he could also be hanging out with this person that day his father saw him coming home late, which probably was not the first time this happened). It wasn't a planned murder per se; Andrew just wasn't receptive when this guy (when they were together in London) made a move on him.
This person panicked, thinking Andrew could say something to his parents, and Andrew's parents knew their parents (they were in the same church in Doncaster) etc. They could be fearing getting 'outed' (the religious family didn't know he was guy), or being charged with enticement of a minor, etc. He could have just choked Andrew while threatening him to keep quiet and suddenly the boy is dead...
A lot of crimes like this are just acts of desperation. They're not really planned, they're just messy and really hard to solve unless the connection between the victim and the possible perpetrator can be established.