Actually, Hugh, a comparison of the UK and US BMW websites shows the following:
In the 3 series, the slower 4 cyl and diesel models are available in the UK, but not the US. The "fastest" 3 series, the 335 petrol has the same acceleration specs for both markets.
In the 6 series, the slower 6 cyl model is available in the UK, but not the US. The "fastest" 6 series, the 650 has the same acceleration specs for both markets.
The M models are essentially the same for both markets.
So, it seems that Dizzy has a good point.
Tom K.