Moggiee,
Please accept my apologies, you are of course correct, the height limitations on the approaches are of course DH.
But from my perspective on the ground, there wouldn't be much point in you commencing an approach if the cloud base was below the DH minimum for the category of approach that you are flying - thereby precluding the probability of achieving the visual cues required to complete a landing. So, ATC tend to be inclined to interpret this minimum as a cloud base minimum, and will not expect you to commence an approach if such conditions prevail. They are not police however, and will not attempt to stop you making an approach. At the end of the day, it is the pilot's decision, ATC will assist with as much relevant/pertinent information as is required.
Likewise with RVR values, they are limiting factors in the decision as to whether or not to commence the approach. I should have been more specific about this in my original post. If the RVR is below the required minimum for the runway and the Category of approach, this will be passed to the pilot and an approach would not be expected, however it is the pilot's decision and he can still elect to fly the approach to DH if so desired. As I mentioned previously, I have seen this happen. Unlike the scenario in the paragraph above, however, an approach in these conditions would result in some paperwork.
This is at odds with UK Absolute Minima legislation and procedures. There is nothing to stop the pilot descending below 1000' if RVR values are below minima for the approach, but as mentioned previously, a measure of professional responsibility is expected.