I've seen it twice now. Felt the same as you on the first viewing, totally disagreed (with myself) on the second.

I don't think it's a detour, because it's certainly necessary, if Simon and co. want to really paint a portrait of a city. It's not "inner city", sure, but the corruption and the bullshit politics of the core really does affect the periphery, too.

Simon's not skirting around the edges; here we see the slow death of the working class as a result of corruption elsewhere. The stats game, the political compromises, the lack of self-honesty in favour of carreerism; all of that leading to the lack of funding, year after year, at the shipyards; and that lack of funding meaning the guys can't get the canals dredged, which means not enough ships are coming in in order to make a living; then you get guys like Ziggy and Nick turning to drugs for their money, and you get guys like Frank trying to keep things going but getting mixed up in horrible shit because their economic needs make them at the mercy of those in power. Bullshit runs downhill, and those at the bottom of the pile suffer most.