[Rosemary]

I have liability insurance; selling direct to the public at the farm gate is much less regulated that say through a retailer. The argument about commercial v smallholder is tenuous and in any case doesn't answer my question - why a different withdrawal period for eggs for my family to eat and eggs to sell?

Just checked the National Office of Animal Health, which is where I should have started 
This is what it says:

"Meat
Birds must not be slaughtered for human consumption during treatment.
Treated birds may be slaughtered for human consumption only after 7 days from the last treatment; the meat withdrawal time is 7 days.

Eggs
There is no withholding period for chickens producing eggs for human consumption when the product is used at the recommended inclusion rate; the egg withdrawal time is zero days."

And no mention of whether the eggs are for sale or home consumption