The aerobatics bit is a slight oddity.
There is nothing to stop a bare minimum PPL from going out and doing aeros in a suitable machine with no training, as there is no legal requirement for formal training.
Giving instruction, however, is aerial work; so if you don't have an FI or CRI, you are not covered by the exemption to perform aerial work. Further to that, an FI or CRI can't give aerobatic instruction unless the limitation has been removed from their licence.
At what point a bit of informal coaching becomes formal instruction may be up to debate, but the giving of aerobatic instruction formally does require an instructor qualification (unless I'm mistaken).