Thoughts on definition for disadvantage of concurrent processing?
I'm studying for the same A Level and have found that deadlock, starvation, race conditions and the overhead affiliated with coordinating simultaneous threads are reasonable, in-depth disadvantages of concurrent processing

Deadlock occurs when each process/thread is waiting for another to finish (meaning that no process is ever able to complete). For example, this happens with cyclic dependencies (e.g. when process A needs B which needs C which needs A).

Starvation is when a process is always denied a resource that it needs, resulting in the process never completing.

Race conditions are when two processes are sharing the same storage location (i.e. variable) and the result of operations depends on the order in which they are ran.

Overhead - there is additional processing overhead to coordinate the individual threads (and combine their results). This can mean, for some problems, a single thread is more efficient than multiple.

Hope I've helped clarify things for anyone who was also confused with this topic,

Josh

Sources:

Cambridge Elevate OCR A Level Textbook

Rensselaer Polytechnic Institute Lecture Notes