Beril, for the purposes of your decision making in 'who not to fly with'...
Stories such as the 'right hand yoghurt' etc have circulated for donkeys years, and among all crew at all airlines. Most of them are probably urban myth, and as i've already stated, in 8 years of flying at 5 different carriers working with literally thousands of different crewmembers, I've only once had cause to question a colleagues integrity (and that was whilst working for a US carrier and not BA).
All airlines have their moments, remember the two seperate stories about Northwest Airlines F/A's putting Gravol or Brandy into a baby's juice?
I've heard more tales of 'putting things into customers food' whilst working in hotel and catering than I have in the airlines, and I've never been witness to any such thing happening at BA. 99% of the colleagues I work with aim to provide a great on board service. The other 1% aim to get through the day with as little work as possible. 0% in my experience would purposefully poison a pilot's (or passengers) food.