He was a Walter Mitty character who knew what he didn't want more than what he wanted. He might have taken to deep sea diving in New Zealand but became bitter when he wasn't accepted on the course and this was just another reason to blame June for dropping him on the head as a baby. He felt his parents owed him (remember the remark to Julie: "they have forfeited their lives for how they have treated me".
Given that he had rejected Nevill as a role model and the relationship with June faltered when he started courting, then she was readmitted to St. Andrew's, he became influenced by the youth role models of the time (Adam Ant spring to mind), and the soft drugs culture which pervaded the London clubs of the 1980s. It's a tragic irony that neither parents listen to their son's needs: he might well have made a success of running a wine bar in a fashionable area of London, delegating the spade work to others as he approached middle age, or would the pretty birds have flown as the money petered out-who can tell?
Walter Mitty is an apt description, Steve. However, in his defense, when all one's choice is so limited almost everything seems like a better alternative. We must remember that other than a good -Ok, expensive- but basic education, he had no qualifications. He had to rely on a plummy accent and Hugh Grant type charm to get by, hence, the unskilled work he gravitated to. It's very likely that the deep sea diving course would have failed, regardless, as self discipline seems not to have been something he benefited from having. The childhood injury may have been a convenient get out. And a stick to beat June with.
I think the Adam Ant era was probably Jeremy's Hey Day. It may have signified a personal freedom for him that he'd never previously experienced. I think that in hosting a wine bar -doing little but looking masculine/effete and glamourous- he may have found his niche, but he would probably have been better at spending money than ensuring that it didn't run out.