Halliday & Matthiessen (2014: 423):
Prepositions are not a sub-class of adverbials; functionally they are related to verbs. But they form groups by modification, in the same way as conjunctions; e.g. right behind, not without, way off as in right behind the door, not without some misgivings, all along the beach, way off the mark. Again, there are more complex forms such as in front of, for the sake of which can be left unanalysed. These are also subject to modification, as in just for the sake of, immediately in front of.