Handled Very Badly
A couple of points:
A number of years ago a pilot with a UK airline had a similar situation where a cabin crew member thought she could smell alcohol on his breath. On that occasion it was well handled and he was politely asked to leave the flight deck for a test. He was completely sober and in fact a tee totaller. Turned out he was on a low or no carb diet which produces a condition known as ketosis. It simply means you have either been having no carbs or too much alcohol. A breathalyser resolved the situation but that pilot was subjected to a lot of abuse until his name was cleared.
In the case of this BA pilot if he is still in lock up then probably he failed the test, but the point is that taking him off by armed Police could have been a gross injustice.
Second point:
The captain on the BA flight can also be in trouble if it is determined that he was willing to get airborne knowing his F/O was unfit to fly. Sorry but the days of covering for crew members with regard to alcohol and drugs are over. No place on the flight deck for this sort of behaviour.