Yes they are. Current alternates direction, but power usually only flows in one direction, from the input terminal (from the bus bar) to the output terminal (that the circuit is wired into).

If the circuit will be supplying power too (e.g. battery storage, an EV and EVSE that supports powering the house from the EV, etc) then you need a bidirectional RCBO.

People with no differential fault protection need not worry about any of this, they'll just be killed when it goes badly wrong.

Source: Am a UK electrician

Example: https://assets.cef.co.uk/downloads/pdg/wylex_nhxs1b32_datash...

EDIT: To say nothing of people with unidirectional electricity meters; plugging these into those setups will get them prosecuted for electricity theft. All SMETS 2 smart meters are bidirectional; you'd best check your meter if it isn't one of those.