My problem is explaining to sales people
That's 75% of my problems too
Reviewing your specific situation:
You've opted / been required to declare the country of origin ("Made in the UK");
You don't appear to have an ingredient that obviously qualifies as the Primary Ingredient based on the 50% threshold, as we've concluded that the water doesn't really fit with this.
The next task is therefore to identify whether there is a Primary Ingredient, as it's possible to have foods for which there isn't one - "Made in the UK with ingredients of different origins" would probably be acceptable in this situation, as it ensures the consumer is not mislead into thinking that both the origin and place of provenance are Britain; it’s in the spirit of Article 26(3)(b) of 1169/2011 even if it’s not directly applicable due to the absence of a Primary Ingredient.
Even if you do have a primary ingredient, a slightly modified version of this may still work – Article 2(b) of 2018/775 makes provision for this type of approach, but you’d possibly also need to be explicit about the Primary Ingredient being included in this. There might be scope to not need to indicate the primary ingredient specifically within this text, as a logical argument could be made that "Made with ingredients of different origins" necessarily includes the Primary Ingredient, but I've not had time to re-read through the reg/guidance this morning so I'd not go so far as to state that it is unequivocally ok, and my gut feel is that the regulators' view is likely to be affected significantly by the context in which it is used.
Alternatively it may be the case that there is a primary ingredient, or indeed more than one primary ingredient – in my Tomato Sauce example, if instead I called it a Tomato & Chilli sauce then arguably both tomato and chilli might be associated with the name of the food by the consumer.
I think this is therefore the approach that I’d take with your sales people – what is it about the name of each product that defines it?
e.g.
Tomato sauce – probably tomato (although there might be an argument that most consumers don't really associate tomato sauce with actual tomatoes )
Brown sauce – quite possibly no Primary Ingredient