BOAC
From a technical point of view they are quite right to use the term 'acceleration'. Acceleration is a change of velocity. It doesn't matter whether its an increase or a decrease (deceleration as you call it), from a technical point of view it is still referred to as 'acceleration'.
The situation you describe earlier, cyclic increase then decrease of attitude and altitude is fugoid oscillation. If the aircraft had been experiencing this then the flat attitude would have placed it near the bottom of one such oscillation, where it would have had a relatively high forward speed. The BEA appear to suggest it had low forward speed at impact.