Sky's "Method for allocating listings in Sky's EPG
" document says:
Where a new service launches, or where it moves between genres, sub-genres or categories, the service will generally be allocated the next available channel number within the genre, sub-genre or category (that is, on a “first-come, first-served” basis). Accordingly, where a channel number is vacated by a service which has either ceased broadcasting or moved to another genre, the vacated channel number will not generally be re-allocated.
Sky may, from time to time, after consulting with broadcasters, fill any unallocated channel numbers within a genre or sub-genre by moving services listed below the unallocated channel numbers up the EPG, or move the services within a genre or sub-genre to a new number range. In so doing, Sky generally will retain the relative order of the services moved. In carrying out such a move Sky may, depending on the circumstances, allow broadcasters to elect to retain the existing channel number for their services.