As stated in the BBC article it's not banning those sexual acts, it's banning depictions (e.g. porn of them). And it's banning these ones in particular because they are considered by the UK Gov to be life threatening.

So it's not about freedom of expression because it's about selling porn. And the gov has a justification as to why these 2 acts because it believes they're dangerous (note they also banned strangulation but that didn't get talked about because few people disagree that strangling someone for pleasure is dangerous).