Spaz/spastic is not an okay word in the US, but it sounds like it's become worse in the UK than here. I would say than anyone in the US who knows better than to use "retard" ought to know better than to use "spaz" or "spastic", though. But a lot of people still use retard, so... (Certainly anyone who's getting offended at potato - which afaik is entirely an internet thing - should know better than to use "spaz" lightly, American or not.)