So on a British BB, the right cup comes loose by turning clockwise. But it still wouldn't budge: Turned out I should start by loosening the left-side cup (turn anti-clockwise, as usual).

As for the BB shell (68mm wide): ID ~33,5mm OD ~40mm. I'll look around to find a set of spacers to find one that matches, before either going medieval on the inner lip with an angle grinder or getting a BB with a longer axle (current: 127mm.)

Thanks all.