Hi Daifly,
On the Airbus 320 we reduce thrust and accelerate at 1500 aal. This usually means a reduction in thrust and pitch. If the nose is not pitched down smoothly it can feel a little 'negative' down the back. If the auto pilot is engaged by this point you should not be able to feel anything as the Airbus is a 1g aeroplane - very passenger friendly. This should apply to all Airbus operators, not just bmi.
bmimainline