Yea, you should always declare all of your formal qualifications.
Although, if there is limited space, concentrate on the core qualifications first (Maths, Science, English), and you'd be alright leaving out 'minor' qualifiications such as RS. But if there's plenty of space then put them all down.
I know someone who is considering applying for the Britannia cadetship, and he got in contact with Airline Appointments (who are handling the applications) and they said that Britannia would likely require proof of all qualifications. That means that you'd have to supply certificates, and if you have a certificate with say, 3 grades C's and an E on it, they're bound to say, "Why wasn't this E included on the application form?"
If you've got what they're looking for then they won't let the lower grades put them off at least giving you an interview. Just remember that if you get rejected, it isn't necessarily because of the lower grades, it just depends what each airline is looking for at that particular time.
Good luck with your application,
John