They've not said whether there was any drug or alcohol abuse, so I don't think you can assume.
They were clearly in poor physical shape when they were found - the man was walking with a branch for support and seemed hungry, tired and in pain. Which is unsurprising after living rough for 2 months in winter.
Once they realised they'd been on the national news, it seems like they didn't want to make any trips to the shops or be seen in public in case they were spotted and reported. They were spotted on cctv going through bins at night looking for food, despite having access to money, so I think they were determined not to be found.
I think their hunger must've forced out of hiding eventually, which is when they were arrested.
Although they seemed determined to keep the baby and not be found, they didn't really have a well thought out plan...they seem to live a very chaotic lifestyle and make astoundingly poor decisions, and its unsurprising their previous children were removed.