At least in the older cars, the BCM has no responsibility for limiting engine power or vehicle speed. Both are exclusively the domain of the ECU (with information provided by the ABS and automatic transmission). I can't think of a reason why they'd move engine management out of the engine control device, so I doubt you'd need to worry about chasing down the BCM (unless it's an anti-theft mechanism that I haven't heard of, which the BCM most definitely is responsible for).
75mph (120km/h) is an odd limitation for the vehicle. The engine is under a fair load at that speed, so having that as a limit for a possibly damaged engine (we call it "limp home mode" ) is quite extreme. Normally the engine in that mode is limited to 2,000rpm, boost is limited as is fuel rail pressure - and that ends up with a top speed of around 80km/h.
So - I wonder if it's something else, like a fuel or air restriction? Can you hold the vehicle in a lower gear and get full power and RPMs up to 4,000 or does it feel like it's giving up around 3,000rpm?
firstly it's most definitely a speed limiter not a power issue. Yes it's certainly been put on by a previous company owner, (I've actually unplugged the telematics they'd fitted) 75mph on the speedo is bang on 70mph on the sat nav so there's the reason. Had a guy who does my re-mapping look in the ECU and it's not been touched.