Thanks for pointing this out. I've implemented a HGV-specific "no left turn" from Park Street onto Market Road (relation/20752422), but I'm not certain whether it will still allow routing via the roundabout. My hope is that the restriction is treated as literal, i.e. "don't route directly from Park Street via specific segment of roundabout onto Market Road", but "do allow routing from/via/to the same ways, when other ways (the rest of the roundabout) are also used in between".