Look at it from his point of view. His plan probably went no further than the escape, so once he was off the lorry it was "Now what?" If he had a real followup plan he probably would have got much further. One of his former school mates was quoted as saying "One thing I will tell you though, he’s not a terrorist. He doesn’t know his arse from his elbow". The fact he even tried to escape (which anyone with sense would know would only make things worse) let alone do the original crime suggests he's probably not the kind who ever thinks ahead.

He had a window of about 2 hours when he could have used public transport to move around (after which too many people would be looking for him) - but during that time he has no money, and given pretty much every station in London has ticket barriers, he's not going to be getting around by train or by bus unless he can lift someone's Oyster card or contactless card without being noticed, and from quotes about his past, apart from the idiotic fake bombs that got him into this mess in the first place, he seems to be a first-time criminal, so successfully pulling off thefts or shoplifting was probably not something he was practised or good at.

After that time he's pretty much limited to travelling on foot in places where he won't get a second look, or at night. He probably found a poorly-secured bicycle on Saturday morning and gambled that people wouldn't give someone cycling down the towpath a second look, and it's faster than walking.