AdVERtisement seems to be the British pronunciation. AdverTISEment, the American way. Ah'm a elderly Suthner....we say ad. No use wasting what little breath I have left on four syllables when I can use one.
AdVERtisment in England although I grew up in Scotland where we always pronounced it adverTISEment...
Similar difference with the Shires...in Scotland it's usually pronounced LanarkSHIRE, RenfrewSHIRE etc..in England although spelled the same way it's pronounced HertfordSHIR, DerbySHIR etc..
Another difference between the American pronunciation and the English is with the suffix Burgh... English (UK as a whole) pronounce for example Edinburgh as Edinburah...Americans pronounce it Edinburrow