I think this varies from company to company.
Some can't start an approach when RVR is less than that required to land.
Others I have come across have started an approach in vis. below minima, commenced final descent to no lower than 1000 feet and if RVR is still below minima, gone around. If, however, at 1000 feet they have visual reference they have gone on and landed regardless of RVR (ie shallow or patchy fog) or obviously if RVR is above minima landed also.
Often on days when RVR is fluctuating it can be a lottery as to whether you get in. If you're lucky enough to be on final approach just after a heavy departure has swirled the fog up, that usually gets the RVR up a bit!
If you have to hold until RVR is at or above minima then you've probably got less chance of getting in than if you can fly an approach and hope it improves in time. This assumes that you are not taking the space in a sequence of an aircraft that can land in the prevailing conditions. If so, you will have to wait for either a natural gap in traffic or a weather improvement.
We ATCOs can't police the system. If you say you can commence an approach below minima and hope for an improvement, then fine. But we do need to know your minima for overall planning needs.
There is a reference to the 1000 foot bit in the AIP somewhere, but I don't know the page.
(correcting typos)
[This message has been edited by cossack (edited 06 October 2000).]