Indeed, a blank sign does not indicate any speed limit.
The issue appears to be that you are taking discrete paragraphs of the regulation out of context, rather than construing the regulation entire.
The operative restriction is contained in regulation 3(1) whereas regulations 3(2) to 3(5) provide further instructions as to how that restriction is to be interpreted.
To take the analysis step-by-step:
The restriction applies to vehicles that are being driven. So far, so good.
We are driving a vehicle.
The definition of such a section is set out in regulation 3(2):
As to (a), let us assume that the road is actually specified (not necessarily guaranteed given the quality of some legislative drafting).
As to (b), the vehicle has passed a speed limit sign indicating a speed limit of 40mph. Regulation 3(5) tells us that
in relation to a vehicle a 'speed limit sign' means a diagram 670 type traffic sign (i.e. a number inside a red circle).
As to (c)(ii), the vehicle has not passed a traffic sign indicating that the national speed limit is in force.
As to (c)(i), the vehicle has only passed a blank speed limit sign. It has not passed a 'speed limit sign' of the diagram 670 type as defined in regulation 3(5).*
Consequently, we may conclude that t
he vehicle is on a section of a road which is subject to a variable speed limit both before and after passing the blank sign.
We must then consider the final part of the restriction:
Regulation 3(4) clarifies that if a speed limit sign indicates no speed limit or indicates that the national speed limit is in force then it "is to be taken as not indicating any speed limit".
It is worthy of note that this paragraph of the regulation is not subject to the caveat "in relation to a vehicle", unlike regulation 3(2), regulation 3(3) and the definition of 'speed limit sign' in regulation 3(5). This means that the speed limit signs to which regulation 3(4) applies are not limited to those that conform to diagram 670 and a completely blank sign can constitute a speed limit sign in this respect.
As such, if the speed limit sign does not indicate a speed limit (i.e. is blank) or indicates that the national speed limit is in force it is to be taken as "not indicating
any speed limit".
This brings us to the logical conclusion that, if no speed limit is being indicated by a speed limit sign, it will not be possible to drive a vehicle in excess of the indicated speed. To put the matter another way, one cannot exceed an indicated speed if no speed has actually been indicated.
* For the avoidance of doubt, regulation 3(4) clarifies that a blank sign is not indicating a speed limit so once again we may conclude that the vehicle has not passed a speed limit sign indicating a speed limit for the purposes of regulation 3(2)(c)(i),