I know this may sound strange, but I wouldn't say "advertisement" at all. I would say "ad" or "commercial" (if on TV) or "poster" or "billboard". I guess if I was reading it I would say ad-VERT-iz-ment, and would assume it was Britspeak, but I may be over-thinking it.

Okay, granted, I almost never SAY "advertisement" in everyday speech because I tend to say "ad", but when speaking more formally or writing, I use the long version, with the non-British pronounciation, however you choose to write that out. (AD-ver-TIZE-ment?)