Gordon,
The BA25 is sometimes late for a number of reasons. Usually it will be caused by the late arrival of the aircraft from LAX or a late tow from the maintenance area. Generally, it is quite a punctual flight though. I wouldn't worry.
The BA26 is nearly always on time because the aircraft has a scheduled ground time in Hong Kong of 9+ hours. Therefore, any moderate delay out of Heathrow can be caught up on the return.
The same applies for JNB and CPT. The return flights are nearly always on time but the outbound flights from LHR are subject to rotational delays etc.