Vancouv... Separation must be maintained at all times so an aircraft will not be instructed to descend to a particular level until the one below has either a) reported vacating the level, or b) is observed on SSR to have left the level, be at least 400 ft from that level and continuing descent.
In the UK the lowest level in the stack varies so is not published (to my knowledge!). Also, it may not necessarily be the lowest aircraft which is next to commence approach. MAny factors decide the order and these are taken into account by ATC in formulating the landing sequence.
Hope that helps...