I have been out of the business of self-employed taxation so I might be wrong but I thought professional aviation training was classified as NVQ5 as therefore, costs could not be offset against PAYE. I think it was the Tory Government who introduced that one!
Usually in order to offset expenses against PAYE, the expenses has to be incurred wholly, exclusively and (this is the killer) necessarily in the course of your employment.
If you're talking about PAYE from a previous/current career whilst training, then I can't see that being allowed. Your taxation has to be relevant to the training you are trying to offset.
When I paid for part of my accountancy exams, I could not offset that against my PAYE but that was 1990.
Cheers
Whirls