It really varies by airline, airport of origin, kind of special meal and cabin class. So some more detail would be helpful to get you better answers.

My experiences with BA recently were in CW:

LHR – HKG I had a Kosher meal which was very good – great in both quality and quantity. Especially the breakfast, this was miles better than BA’s own. Even the cabin crew said the meals looked better.

SFO – LHR I had an Asian Vegetarian meal (AVML). I thought I’d ordered Kosher, they told me I’d ordered Halal, and with BA from the US apparently all halal meals are served as AVML. The food was pretty good actually, though the breads were overly dried out.

In both cases presentation, quality and quantity were definitely J-class.

It’s worth searching the BAEC forum for other threads on this.