To Brits, yes, "a" flannel is a cloth for washing, but flannel is a type of cloth, woven, with a nap. I believe lumberjacks and grunge fans wear check shirts made of it.
Flannel, is of course, the cloth in American English as well. Using "flannel" as a noun rather than an adjective for "flannel shirt" (exactly the type you describe) started with grunge. I'd use a photo to illustrate, but it might be seen as patronizing.
As for spastic, it's rarely used in the US outside of medical diagnosis. For instance, someone can have a "spastic colon" or someone might have "spastic quadriplegia", but rarely would you hear someone call a mentally or physically disabled person a "spastic" or a "spaz", especially in a derogatory way. The difference has caused some widely publicised incidents, one with Tiger Woods, IIRC. Anyone who corrected an American who was using it is probably familiar with the UK use of the word, at least in my experience from working as a counsellor for the disabled in the States. Now, after living in the UK, I wouldn't return to using the word, even in the States.