Channel number allocation is actually very straight forward on Sky.
Yes the first 5 channels of any operators EPG are reserved for BBC1/ITV etc.
But after that as far as Sky’s EPG is concerned it is first come first served for each channel in each category.
If a new channel is added to say the entertainment section it will take the channel number three places after that of the last current channel (say the last channel in entertainment was no 268 you would get 271.) The reason you get a channel three spaces after the last channel is that each channel has two ‘reserved’ numbers after it, this means that if you launch another channel, or a +1 type channel you can use the number(s) immediately after your channel. This is why Sky1 Mix and UK Gold 2 etc all have places immediately after their parent channel, but if UK Gold was to launch another channel it would appear on the end of the entertainment section as they have now used up their three spaces at 109+
So your friend with the film channel would get the channel number three spaces after the last current film channel.