hiya, i used to work for the AA before i got a proper job.
NCD can save you up to 70% on your policy, bearing in mind if you're hit by an uninsured driver (1 in 12 in the UK) you'll lose your NCB hence the discount, it can really push up your policy, i've seen it tack on 600 quid to a fiesta before!
Legal protection is also pretty necessary (i have both on my policy even though its not with the AA) if you hit someone we're pretty litigious nowadays and a whiplash claim can cost you 5 grand! obviously the more people in the car, multiply accordingly.....
personally for the 40 quid it might cost you for both i would pay it just in case, its like breakdown cover, you probably won't need it but you'll know about it if you don't have it!
hope this helps!