I lived at home when I went to uni (late 90s). Not in London though.
Positives: I had plenty of money, no debt and my own car. I had a part-time job and a comfortable place to study. And a cat! My folks treated me like an adult and I behaved like one, by and large. I grew my own independence in a healthy way. I left home at 21, and I was ready. Not sure I was at 18.
Negatives: I did miss out on some of the 'experience' and as a young gay man I might have had a slightly easier time a couple of years sooner than I did. (I say this for context; clearly this isn't relevant to your lad, since he has a girlfriend.) But to be honest, even for the people I knew who stayed in halls, all that impromptu party stuff and arguments over who buys the coffee got really old after about a month anyway.
That said, I did make some uni friends on my course. I also had a good, supportive and functional social group with people I worked with, outside of uni. So much so that some of the cliqueyness of halls life seemed a bit immature to me, even at 18.
I also came out of university with a First and zero debt. Everyone else was tens of thousands in the hole. So I definitely gained there.
Also I moved out of home fairly quickly afterwards. In fact I went travelling and lived abroad for almost three years. So it's not like I was tied to the apron strings.
What made a difference to me was that my parents adjusted how they treated me once I stated uni. I cooked my own meals, did my own laundry and sorted out 'lifemin' stuff like getting my car serviced. I no longer had a curfew so if I wanted to stay out all weekend (it was the 90s...
) I could do that.
All in all, I don't regret it. And if I was choosing again, I'd probably do the same.