Was it a re-badged oriental bike?
No, I think these bikes were assembled in the UK, the parts including the engine were mostly of Italian manufacture, although there were BSA 125 and 175 Tracker models that used the Yamaha DT125/175 engines.
Correction:
I did say previously that the Boxer was "an NVT product" although by the time that the Boxer (later called the GT50) was being produced I think BSA was trading as an independent company once again, and still is, as part of the BSA-Regal Group.
http://www.bsa-regal.co.uk/