If that was a concern he wouldn't have taken the train at all. Dovercourt is the stop after Harwich International and before Harwich Town. All three are within walking distance.
It seems John wouldn't have much knowledge of Harwich as he seems to have no links there. It is however in close proximity to Colchester, a well known military hub. That being said, John was on the train heading to Colchester (Bury St Edmunds to Manningtree carries on into Colchester) He however got off that train and got on one heading East to Harwich.
I see a few possibilities
It seems John wouldn't have much knowledge of Harwich as he seems to have no links there. It is however in close proximity to Colchester, a well known military hub. That being said, John was on the train heading to Colchester (Bury St Edmunds to Manningtree carries on into Colchester) He however got off that train and got on one heading East to Harwich. This is a possibility, however it seems unlikely.
- He went into mainland Europe
It seems unlikely he would have tried to leave the country, he had very little money on him. Not enough for the ferry into Europe. It surely would have been flagged up on passport control if he had tried to leave. Also the ferries have CCTV (although the police seem to be rather slow at gathering footage, so it's still a slim possibility).
This is possible, it is however a odd place to go. If he wanted to live rough he would have gone to a larger city. Far less likely to stand out. Considering he lived in Stevenage, he could have easily gone into London and vanished. I've also not read anything about him taking camping equipment such as sleeping bags or tents. So I doubt he's in the woods or anything like that. He also didn't have enough money on him to purchase any equipment.
Hopefully this isn't the case, it is however the one that adds up the most. Harwich is the nearest part of coastland to Stevenage. John was also a navy man, he may have decided the sea was his chosen way.