  This is the second of two papers in which we study real and virtual
photon-proton scattering in a nonperturbative framework. In the first paper we
have identified the leading contributions to this process at high energies and
have derived expressions for them which take into account the renormalisation
of the photon-quark-antiquark vertex. In the present paper we investigate the
approximations and assumptions that are necessary to obtain the dipole model of
high energy scattering from the results derived in the first paper. We discuss
the gauge invariance of different contributions to the scattering amplitude and
point out some subtleties related to gauge invariance in the correct definition
of a perturbative photon wave function. As a phenomenological consequence of
the dipole picture we derive a bound on the ratio of the cross sections for
longitudinally and transversely polarised photons. This bound is independent of
any particular model for the dipole-proton cross section and allows one to test
the validity of the assumptions leading to the dipole picture in particular at
low photon virtualities. We conclude that the naive dipole model formula should
be supplemented by two additional terms which can potentially become large at
small photon virtualities.
