That's not entirely true.
Sky have agreements in place for various channels to use slots higher up the EPG
, and this includes TLC which is on 125 and would otherwise be on 145.
I'm aware many companies swap channels around within their own EPG allocations, and Sky agreed to a swap between Discovery's TLC and Challenge to move t up to a higher slot when it launched.
However, most of the Discovery channels occupy the highest slots in the Docs genre, and Sky don't have any they own outright (such as the jointly owned History) that they could allow a lower-placed Discovery channel to occupy as a swap.
The point I was making, was that Sky have no authority to randomly move channels higher up the EPG.
Even when the EPG is reorganised to remove gaps, it's done in strict order, so that no channel gains an advantage that they didn't previously have.
The only time this happens, is if a new PSB launches, and that gets the 'next highest' vacant slot (eg when BBC HD and BBC Alba launched). But that's not likely to ever happen again.