In practice the 1800m viz requirement is not a problem. I don't recall it ever being a problem in 5 years.
This may be because viz is rarely that bad. 1800m is serious haze/mist and in those conditions one tends to not have a cloudbase problem. 3000m is common (like today) but 1800m is not common.
The biggest problem is cloudbase below the MDH or DH. Much of the UK is not flyable because of this, much of the year, and that includes most airfields with an IAP too. Only an ILS really cracks it.