An interesting video.
I believe that the purity of the
denarius and
antoninianus were much the same at around 50% silver in this period (180-220 AD). However, the ant was one and a half times the
weight of the
denarius and twice it's value. However I do not believe that this has anything to do with the reason why the forgers chose
denarii as their model. After all they could have made ants more cheaply.
The
antoninianus was introduced by
Caracalla but was discontinued by
Elagabalus. Coins take some time to get into full circulation and it's probable that the coin
had little impact in far away
Britain. It's much more likely that the forgers chose the
denarius simply because it was familiar with the local population and wouldn't stand out like a sore thumb.
Of course we don't actually know what metal the
forgeries were made of. It could have been bronze -'
limes'
denarii perhaps. There is a small piece about the finds here
http://www.wildyorkshire.co.uk/naturediary/docs/2002/10/28.html including a drawing of a mould for
Severus Alexander.