The Cambridge book says chunks and mince. True, they're the 'purest' of the Quorns and have high protein and low carb content. This is why they're the listed products. Remember there are beef style chunks now, also.
By personal choice I have had the plain Quorn fillets (not the breadcrumbed ones!), the slices, plain burgers (you can get them in bags of 6) and plain sausages whilst doing Cambridge - and with little ill effect on my weight loss. I made sure to buy the Quorn products with the lowest carb content I could - some of the sausages are higher in carbs than others so I compared packets and avoided those. A calculator and the nutritional info panel was essential, and I used both to monitor my intake of these other Quorn products.
Why did I choose to do this? - I would have been bored with just the chunks and mince, and boredom would have been a potential trigger for me to go off plan big style. But I also understand the importance of the calorie intake and low carb-ness needed for CD to be effective, so worked out a compromise.
Note this is not a green light recommendation! - simply my experience and choice. If anyone else chooses to use Quorn products other than those listed, it should be their reasoned choice.