Dress code - aptitude tests
As someone who has represented the NATS operational side of ATC at aptitude tests AND one to one interviews, I consider smart casual dress code for the latter is correct...However I'm not sure that I think jeans are appropriate in context of job applications! Even if it is only written tests and not interview. Jeans may be smart casual for a night out with pals but in this context they would say to me that perhaps the person concerned isn't too bothered about this job in the first place!!
Hope that helps.