In the UK all STAR and SID charts in relation to speed restrictions state "unless otherwise advised by ATC". If you have been given speed 280kts then you have been otherwise advised, most ATCO's where I work always say "speed ***kts until advised".
Agree with FlyinWithoutWings though, IF IN DOUBT ASK ATC as often the safety (ie separation) is going to be compromised if you do not fully comply, which only leads to more nasty forms through the postal system.