Default [Q] GN3 carrier-specific variants

I recently bought a refurbished GN3 in the UK. It was marketed as an N9005 but it turned out to actually be an N900A - the AT&T variant. Obviously all the AT&T guff is not much use to me, and I'm having difficulty making it install the 4.4.2 update from the UK.

Does anyone know if it's possible to install the generic N9005 ROM on this phone? Any pointers to how to do it?