I think other people are more likely to brand Bernie as a socialist (oh horrors!! A Socialist! Where's McCarthy when you need him??) than Bernie is--he describes himself as a Democratic socialist and there is a big difference--democratic socialists support capitalism, for one thing. So there is no disconnect between Democratic socialism and what the prime minister calls his own country: a market economy.
He likes to call himself a socialist, why, who knows, he does have a somewhat activist past so maybe he likes identifying with radicalism. Maybe it's pre-emptive, oh well they're just going to call me a pinko anyway (no matter how politically illiterate this may be and when a guy is going off blabbing about national socialism being socialism like in that article you know they are utterly political illterate). But I think he just likes the label. He's a social Democrat, he's also much like an FDR Democrat. Does anyone anywhere use "Democratic socialist" that way as opposed to "social Democrat?". "Democratic socialist" isn't a term in common use I don't think, but I'd tend to think it means something like worker co-ops, which are socialist, and democratic (or aim to be).