This may seem like a strange question, but why, officially, are metric signs actually manufactured at all?
I can understand ignorant councils ordering and erecting them, but someone, somewhere is ordering that they be made, fully aware that they are illegal. Because the erection of these signs is illegal per se, there is no 'truthful' reason for their manufacture in the first place. We know why it's being done, but their admitting to trying to implement creeping metrication on the quiet is rather unlikely. He can hardly say that they are for future metrication, as to produce them before that date is just a waste of time, not to mention storage capacity.

Surely if this/these person/s can be questioned, the manufacture can be stopped, as, logically, producing these signs is just a waste of money, and I see no way that those responsible could argue otherwise.

The question 'why' is a very good one. I think the answer goes something like this:

The Department of Transport wants to convert to kilometres ASAP. So they give a nod and a wink to local authorities.
Local authorities are already in 'metric mode', measuring things, advertising them in metric, telling everyone else like traders and architects to go metric etc. Maybe there are some officials deliberately trying to get as many metric signs up as possible.
Sometimes there are 'development' or 'improvement' schemes e.g funds for new pedestrian signs which _may_ well have a condition attached that metric signs are erected.
Signs manufacturers gain every way by erecting metric signs. It paves the way for eventual metrication, from which they will make a small fortune - and in the meantime every time BWMA or UKIP or someone else forces a local authority to replace a metric sign with an Imperial one - hey presto! - still more business!
We are the fools as all of this is going on using _our_ money.

Tony