This is a quote from CAP 413 which deals with the UK. In simple terms there is nothing stopping the pilot carrying out the approach, but expect to find yourself down at the CAA office explaining yourself.
Another reason could be that this was the RVR they needed for a CAT 1 approach (although very high if the case), so maybe they carried out a CAT 2 approach instead which had a lower RVR requirment.
7.3.8 In the UK, there is an approach ban which states that a pilot may not continue an instrument approach beyond the outer marker or equivalent position, if the reported RVR, or at aerodromes where RVR measurements are not taken or available, the visibility, is below the minimum specified for that approach. Essentially, this means that a pilot may not descend below 1,000 feet above the aerodrome when these conditions exist. This RVR/visibility is known as an 'absolute minimum'.
7.3.9 Should a pilot indicate that he or she intends to commence an instrument approach when the reported RVR/visibility is less than the notified 'absolute minimum' value, the controller should inform the pilot using the following RTF phraseology:
BIGJET 347, you are advised that the
current RVR/visibility is (number)
metres which is below the absolute
minimum for a (name) approach to
runway (number). What are your
intentions?
7.3.10 If the pilot states that he still intends to continue the approach below 1000 ft above aerodrome level, the controller shall inform the pilot.
BIGJET 347, if you continue the
approach and descend below 1000
feet above aerodrome level, it is
believed that you will be
contravening UK legislation and I
shall be required to report the facts.
Acknowledge
7.3.11 This shall be followed at the appropriate times by the following transmissions
BIGJET 347, there is no known
traffic to affect you making a (name)
approach to runway 30
BIGJET 347, there is no known
traffic to affect you landing, surface
wind 280 degrees 16 knots