Just double checking, you are saying reduced competition and scale economics are "stupid edge cases"?

I’m saying that using a courtroom to decide the definition of a biscuit indicates a problem with sales tax legislation.

Flat rate sales tax has its problems, but avoiding Jaffa cake situations is entirely desirable.

Agreed, complex policies open the door for unintended consequences and have higher enforcement cost. If you do a sales tax or VAT it should be flat. If anyone thinks that it will be regressive or put poor people at a disadvantage, the answer IMO is UBI, even a small one can make up for sales tax on food while avoiding the deadweight loss from nonsense like that Jaffa cake nonsense.