Seems to happen with most films now - think they get a 7 day window to repeat a film without additional charges.
That's why most films end up on iPlayer for 7 days - most british (Paddington 2, Shaun the Sheep) films even get the usual 29 days.
Being a British film has nothing to do with it. If it's a BBC Films production then the BBC will have all the rights to it anyway. Beyond that it's all in the agreements they have for it from the distributor. Both those films are distributed by StudioCanal and they're recent films so it's probably easier to get 30 days for them on the iPlayer.
Sound of Music is a exception because (it says here) the BBC bought the rights to it back in the 1970s and they've had it ever since but it's still only seven days on iPlayer as that's the agreement they have for it.