Ah yes, so it is - a product from Harkers by the looks of things that contains ivermectin...
They are selling it under the Small Animal Exemption Scheme which covers small domestic pets not intended for food use. In other words, not chickens

11 Looks like a product used on Pigeons who also suffer from many similar mite / lice problems.
The way it works is if I said yes that would be suitable for say Scaly Leg Mites or Lice for example, I could get into trouble - because it's not licensed for chickens - you would need a vet to prescribe it to you.
Medicines must be prescribed to pets under something called the cascade. The cascade is a legal guideline which states that authorised medicines must be used where possible but gives vets the flexibility to use other medications in situations where an authorised product is not available.
This is how the cascade works:
Ideally use a drug licensed for a specific condition in a specific species
If not...
Use a drug authorised in the UK for use in another species OR for a different condition in the same species
If not...
Use a medicine authorised for human use OR a medicine from another country authorised for that use OR a medicine prepared by a vet, pharmacist or Suitably Qualified Person (SQP) holding a manufacturer's licence.
You find this alot with chickens - because they produce eggs and meat for consuption, you can guess what a huge cost it must be to test products so most companies tend to go for the Small Animal Exemption route and we're left with having to go to a vet to get a product prescribed.
That said, many just see what others use or have had prescribed in the past and use some common sense.
I think I'll put some of this info into an FAQ in this forum it will be useful ;-)