The law doesn't define the material which a knife is made from, so (theoretically at least) plastic knives are included in the same rules as metal ones.
The law only refers to 'selling' knives, rather than to 'supplying' them but, since takeaway food establishments don't give away plastic knives to people who don't purchase food, it's likely that a court would rule that the provision of a knife constituted part of the sale of the food. (i.e. legally, the plastic knives are 'sold' rather than 'given away').
My view is that the provision of plastic knives, by takeaway establishments, to persons under the age of 16 might well conflict with the law but it's unlikely that either the police or trading standards authorities would intervene and, even if they did, it's even less likely that the Crown Prosecution Service would take the case to court.
http://www.opsi.gov.uk/acts/acts1996/ukpga_199 60026_en_1#l1g6
Chris