I do hate it when amateur lawyers witter on about legal requirements, particularly if their wittering is
Based on decades of experience and training.
Far better based on the law, in my professional opinion (profession being law professor and part-time practitioner).
The law in question is the Air Navigation Order 2009 (as amended) paragraph 16(7) which reads:
An aircraft registered in the United Kingdom with an EASA certificate of airworthiness must not fly otherwise than in accordance with any conditions or limitations contained in its flight manual unless otherwise permitted by the CAA.
So a defect in any piece of equipment whose proper functioning is not mentioned as a condition or limitation in the flight manual is not unairworthy, as a matter of law.
This leads to interesting consequences - for example, an aircraft with a broken elevator is still, as a matter of law, airworthy.
Once we move beyond the legal definition, we come to the more normal use of airworthy - is it sensible to fly it? With a broken elevator, no, and there are more general legal requirements (not endangering others, in particular) which would be applicable. But these are all matters of judgment, ultimately by a judge.
So, if something not falling within paragraph 16(7) is defective, you the pilot must form a judgment as to whether you consider you should fly the aircraft. Part of your deliberations might include the judgment a judge would come to if you plummet, in the traditional manner, into a school full of visiting puppies and kittens.
Anyone who says that a piece of unserviceable kit (outside paragraph 16(7)) automatically makes the aircraft unairworthy in the legal sense is simply acting the fool. As an example, my glider has flight controls which need to be connected, by me, once I have rigged it. These connections have safety pins (hurrah!). And the pins are attached by pieces of string, to stop me dropping them into the bottom of the fuselage. If the string breaks, my glider is nonetheless airworthy. Without the safety pin it is
still airworthy in the legal sense, because the handbook does not require the pins to be fitted (just fittable), though I would decline to fly it in that condition because it wouldn't be airworthy in the non-legal sense.
[Edit: and I see that while I was writing this Cobalt said much the same, though less pompously]