Is the marketing statement "our cheese is made of 100% plant ingredients" false advertising and is it legal?
In EU there is legislation against using the name "cheese" for non.diary products. This was unsuccessfully challenged in 2017 by company Tofutown:
https://curia.europa.eu/jcms/upload/docs/application/pdf/2017-06/cp170063de.pdf [press release, DE]
https://curia.europa.eu/juris/documents.jsf?num=C-422/16 [full case, EN]
The government guidelines on naming food products states that if an ingredient is different to what consumers expect, it must be made clear by either:
including the ingredient as part of the product’s name
stating the ingredient close to the product’s name on the label
For example, if a pesto sauce has been made with parsley instead of the traditional basil, the product must either be:
called ‘parsley pesto sauce’
have the ingredient ‘parsley’ stated next to or directly under the product name
So, it seems that the phrases like "our cheese is made of 100% plant ingredients" or "plant based cheese" (without quotation marks or asterisks) are compliant with these conditions.
As an aside, the names of 15 cheeses are legally protected, but this relates to their geographical origins - not the product’s actual ingredients.
I'll limit my answer to the specific example in the question:
our cheese is made of 100% plant ingredients
This is not false advertising or a deceptive trade practice. It is legal. This is because the statement itself is in the form of a definition of "our cheese", and discloses on its face in a way that avoids any possibility of confusion that it is not "cheese" as defined as an animal. milk curd product.
It is also not false advertising because it uses the word "cheese" in one of its accepted plain meanings.
More generally, words almost always have multiple meanings in the English language. For example, the Merriam-Webster dictionary has the following six different meanings (of which definition 2 of entry 1 is the sense in which it is used in the quoted material in the question):
cheese noun (1), often attributive \ ˈchēz \
Definition of cheese (Entry 1 of 3)
1a: a food consisting of the coagulated, compressed, and usually ripened curd of milk separated from the whey
b: an often cylindrical cake of this food
2: something resembling cheese in shape or consistency
3: something cheap or shabby : cheesy material //cinematic cheese
Definition of cheese (Entry 2 of 3) transitive verb
: to put an end to : STOP cheese it —used in the imperative as a warning of danger cheese it, the cops
cheese noun (2)
Definition of cheese (Entry 3 of 3) slang
: someone important
I am not a lawyer (and thus I may be misinterpreting the regulations), and I do not know if the UK law in this area has changed since leaving the EU, but it appears that this may not be legal due to EU legislation, most of which I believe was transcribed directly into UK law.
Regulation (EU) No 1308/2013 defines "Milk" as meaning exclusively the normal mammary secretion obtained from one or more milkings without either addition thereto or extraction therefrom.
Regulation (EU) No 1308/2013 reserves the following names exclusively for milk products
(a) the following names used at all stages of marketing:
... cheese ...
Reading the page further, there is a link to Decision 2010/791/EU which permits an exemption for "Fruit cheese (for example, lemon cheese, Damson cheese)"
However, there does appear to be another exception which may be applicable, although the legalese is so dense in this section I can't fully follow it:
As an exception, this principle is not applicable to the description of products the exact nature of which is known because of traditional use and/or when the designations are clearly used to describe a characteristic quality of the product.
This question is currently tagged united-kingdom.
In British English the words "cheese" or "curd" simply refer to different stages of a thickened or coagulated liquid.
While the most common type of "cheese" with which most people are familiar is a dairy or milk cheese, there are other types of cheeses and curds available — most notably various fruit preserves, separate and distinct from jams, marmalades, or chutneys. For example, Lemon Curd, or Damson Cheese.
As such, the use of the word "cheese" to refer to a a product produced by coagulating plant products is just as valid as using the word "bicycle" to refer to a Penny-Farthing: it's simply not what you are used to, or instinctively think of when you hear the word.