I think there are a few misconceptions about what the Harrier would be used for if Cosford were to obtain some for use in the training environment. The intention is to use them for a final consolidation exercise and it is beneficial if the task being done is something that has not been encountered before. This is perfectly fair and acceptable provided you recognise what it is you are assessing and what you are not, what you can expect the trainees to be able to do and what you are going to have to supervise, e.g. the connection of the lifting kit and the lifting operations.
It is the process that is important, not the actual task itself, i.e. the way the trainees apply the training they have been given, the way they approach the task, think about the task, access information, document their work, demonstrate their hand skills, demonstrate their ability to work safely and so on. The unconventional nature of the Harrier is irrelevant, yes a Jag is similar to a Tornado but its not similar to a Herc or a VC10 or a Timmy is it, so its not representative of those platforms, but it doesn't have to be. At the end of the day lifting a wing off and the engine out of a Harrier is just an aircraft engineering task.