Your pretty much there. You CAN commence an instrument approach regardless of the RVR/VIS but CANNOT continue past the OM or equivalent position (1,000' above the aerodrome) unless the RVR/VIS is within your limits.
Where the RVR is not available then you can derive an RVR by converting the VIS.
As you say once past the OM or equivalent position and the RVR drops below minima then you can essentially "have a look". If you get the required lights etc then you can land otherwise you go around.
It is particularly important to read the NOTAMS in low viz operations as certain inop items can have a significant effect on the landing minimums eg: centreline lights u/s etc. A certain loco found this out only a few years ago at Stansted. JAA have tables of Aerodrome Operating Minimums (AOM) for these which are in my companies Jeppy text manuals and available for reference in the ships library plus in ops back on the ground before you head off. You will probably find them in the UK AIP which you can download off the net or find at your flying school.