A few thoughts -
The regulations put in an extension of 6months to allow for overlap in design to fitting of equipment, where this was exceeded by 3 or 4 weeks due to delays etc then I'm sure it can be argued that the original plastic CU can be fitted but we are now 6months down the line from the cut off date so the designed prior to july 2015 really is no longer an excuse, a meeting with the client should have been made and the situation discussed, the plastic CU can still be used as long as its boxed in a fire proof surround which will be an extra at the clients expense.
On the other side of the coin - plastic has been used for decades and if fitted correctly and terminals are tight then no real hazards should exist, this however is not a reason for not complying to the reg's AM'3 which I personally believe have been brought in as a knee jerk reaction due to a drop in standards in the trade and a marked reduction in quality of CU's due to mass production and competition.