DOC 4444
Item 5.3.3.6:
"In so far practicable, cruising levels of aircraft flying to the same destination shall be assigned in a manner that will be correct for an approach sequence at destination."
Item 5.3.3.7:
"An aircraft at a cruising level shall normally have priority over other aircraft requesting that cruising level. When two or more aircraft are at the same cruising level, the preceeding aircraft shall normally have priority."
I am sure most controllers would use vectors to handle crossing traffic, if they have radar that is, but there are lots of situations where it's easier to change levels, could be there's no room for vectoring, trafficload, no radar and so on.