Sharpshot: You can ask for RVR values at any time, but ATC might not actually be recording them (if there is no IRVR equipment they need to send a bloke out to a designated spot by the runway to count runway lights..) above certain met visibility. If the reported visibility or RVR/IRVR values meet your minima then you can legally descend below 1000ft QFE and follow the approach to DA/DH/MDA/MDH as appropriate. If you get down there an can't see anything, then you have to execute the missed approach. Beware of being caught in the trap of being able to see the lights and land and then finding the official RVR is below the airport minima. Then you'll be in trouble!!
Your question about going to MDH/MDA and flying level is quite OK, provided you start the Missed Aproach no later than the Missed Approach Point (shown on the approach plate). Indeed, some people advocate descending to MDA/MDH fairly quickly after passing the FAF and flying level until you either see the lights or whatever or until you reach MAP, whichever comes first.
Remember, however, that if you are on a precision approach, you must commmence a Missed Approach IMMEDIATELY if you reach DA/DH and do not have the required visual reference to land.