Only for quantities of <2m^3. I suspect this means retail outlets such as garage forecourts and DIY sheds.
It's just about impossible to enforce for sales where the vendor brings the wood to the home although I think most firewood merchants are aware of the law and will take steps to be seen to be complying.
It's impossible to check. How does anyone know whether the wood I'm burning now is from a kiln dried load, a bulk load of >2m^3 or given to me by my friendly local gardener. And the storage once it's been delivered makes a huge difference to its subsequent moisture content.
In the interests of not blocking up my chimney with crap I burn the driest wood available to me and it gets stored for 2 years if it wasn't delivered kiln dried. But you have only my word for that.