I assume you mean (in the text I emboldened) to have a pre-test garage check. Which I did. The mechanic noted two things that might have caused it to fail, the worn ball joint and a worn drop link, aka: anti-roll bar link. It failed the test just on the worn ball joint but, just to be on the safe side, I asked the mechanic to also replace the drop link when he was doing the repairs.
I deliberately put the word "legal" in italics in my original message, as I actually was legal between failing the test and presenting it for a retest within 30 days of failing the initial test. Had the fault been considered extremely serious by the test centre, I would not have been allowed to drive away, but arrange it to be lifted (not driven) to a repair centre, or else just decide to scrap the car.
I trust my mechanic, having dealt with him (and his father before him) for over 20 years now. Had the faults been serious he would have insisted on repairing them before the test. His thoughts were to let them fail it on one or both of the faults as the car is now twelve years old, so the chances were that if he had repaired them beforehand, they would just have found something else to fail it on. I was prepared to trade it in for a newer model had he (or the test centre) found faults that were going to cost too much to put right.