Default unit of measures

When setting up a product you can specify the default unit of measure for Sales, purchase, stock and these can be different, so I can have a different sales unit e.g. Each to my purchase unit e.g. Box or Pallet and the system can convert between these using unit of measure conversions, so all good so far.

When using product masters and enabling unit of measure conversions for variants all the unit of measures for sales, purchase & stock have to be the same, so the default unit of measures on my orders end up being 'Box' for sales when I want them to be each.

I need to enable UoM conversions as my products can be different sizes (so boxes can contain different numbers) 

Any ideas on how to get around this restriction?