In redundancy the job that the employee is doing "disappears". You can still take on new staff but not to do the work the redundant employee was doing.
However, if business conditions change, ie if BA increases flying capacity or suddenly needs more crew, they are legally able to start recruiting crew again. There is no time limit set by law, they would just have to demonstrate that the need for more crew had arisen.
Zeb (studying employment law at uni!!)