I don't think it only covers genocide but international law beyond things like genocide is mostly just the treaties between nations and the whole point of Brexit was to prevent being bound by treaties that prevented autonomy.
There's no treaty that says the British Government must grant a license to hold a football game at a certain ground at a certain time. Under the EU they might be done under some sort of monopoly or state involvement rule but they no longer exist.
Of course they can break existing laws if they really want to, but its still a serious step to take.
For one, what if the law they come up with contradicts their "competition law"(which is essentially still the same as EU, btw), then they would have to make changes to the "competition law" to accomondate their new law.
All im saying, law is not straight forward, it can get quite...complicated, Brexit isnt going to change that.