Well the other two replies said that you couldn't, which holds true for students at my school, but I'll explain why.
Spec is in a sense 'harder' than Methods because it builds upon assumed knowledge from Methods. As such, most schools would not let you do Spec without Methods because it's such a high chance that you'd fail the subject and they can't risk bad scores etc. I don't know if courses accept only Spec (I think so?), but it's assumed you've done Methods if you use Spec as a prereq.
Why would you choose Spec instead of Methods? Just curious.
I did Methods as an accel. subject last year and I am doing Spec this year. I am fortunate in that I don't have to do two Maths at once, and as many and many of my accel friends have commented: we would mostly likely not have done Spec at all if we'd had to do two Maths together. So I'm wondering why you want to do a 'harder' subject without the stepping stone. It's more content, more work and especially a pain if you don't know content from Methods.
Example: An entire chapter in Methods is covered in 3 pages of the Spec textbook. It's all assumed knowledge and revision.