There is a very, very basic failing that many of the Meldrews on this thread are conveniently ignoring.
The law is there for a purpose, it is intended to be precise for very obvious reasons. Thus unless there is a specific provision which accurately describes the situation it does not apply - the activity is 'legal'.
Strangely this is why laws are actually drafted with a great deal of care.
The complete failure of a direct reference to not using portable devices in an aircraft does show the difficulty the Meldrews face. If may be illegal to wash a hackney cab in the street on a Sunday but not it is not illegal to use portable devices in a light aircraft.
So this is the usual huffing and puffing!