Would need to check the definition of "damaged" - if does not say it has to be caused by a third party and there is no reference to wear and tear and a garage confirms it is 'broken', then on the face of it they should pay for the replacement less £100, IMHO.
If the brakes on an otherwise road-worthy car fail and it crashes, the damage would be covered by a Comprehensive policy and in that example there is obviously 'wear and tear', hardly something the owner can avoid. So if an Insurance Company chooses to protect an item that means the insured cannot use their car when it fails - only right that the Company pays out in my book. But then there is always the dreaded small print.