How to safely cap a wire coming from a wall
I would terminate the wires in a terminal block, coil the cable up inside the backbox and fit a blanking cover.

If there isn't a backbox fit I'd one or fit a surface pattress.

I believe it is required that the routing of cable through walls be made visually evident by the presence of visible surface fittings. If you intend to remove the visible surface fittings, you must completely extract the hidden unused cables from within the walls.