Executive discretion
In common law jurisdictions the executive has the prerogative to enforce or not enforce particular laws. The courts will not interfere provided that prerogative is executed for proper purposes.
It is that prerogative that allows governments to have a “blitz” on speeding or seatbelts where they direct more resources at a particular problem for a short time. It’s also that prerogative that allows a police officer to issue a caution rather than having to arrest every lawbreaker they see.
In general, courts will not allow the prosecution of a person who was given permission by the executive to break the law: it’s seen as an abuse of power to say “sure, you can do that” followed by “gotcha!”
This is a general power but, in this particular case, the FSA has the explicit power to make emergency orders.
What “public good”?
The link is explicit as to why it was issued. Further the liked risk assessment says:
It is highly unlikely that industry will be able to re-label products as quickly as oil substitutions may occur, which could lead to the presence of mis-labelled products on the market.
Basically, producers have a bunch of packaging that says “sunflower oil” and no time to get packaging that says “rapeseed oil”.