I'd argue against the Mustang or the lower Peugeots (Lazer?) being BSO's. They were bikes built to a budget which acted as a gateway into our great sport for children up and down the country. They were heavy and basic but pretty robust.
Still remember my first ever go on an MTB (ATB?), a mate's Mustang with 15 gears, and I was hooked straight away. Coming from a BMX my mind was blown by the uphill ability in the local park when stuck in the lowest gear. Far from being a BSO it was actually the halo bike in our village!
Real BSO's are the cheap tat that followed, the type with brakes so cheap they bent rather than stop you and metal that snaps on first contact with rough terrain because it's so soft. The kit churned out to look like an MTB but that's it.