The ants they sell in the UK should be fine for your lizard. There are many folks in the UK are other areas that use these ants for horned lizards with good success. I believe they are lasius niger if I remember correctly.
Do not use acids with your lizards. This is an old claim and has not been proven or documented by any reliable sources that I am aware of. It is a long standing argument that is often quoted but never proven.

You can find cornutum specific husbandry info on the phrynosoma.org website. Look under the "Husbandry" links. It is best to read all the husbandry links as they all have differing information and it's all valuable. The hernandesi info would be the least important in your particular case.
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Phrynosoma.Org
Phrynosoma.Com