First my disclaimer...Advice given in this website is correct to the best of my knowledge and is my interpretation of a complex subject. You should seek professional legal advice if you need absolute clarification of the laws relating to health and safety.
A PAT test in itself is not a legal requirement, however obeying the health & safety laws is. A PAT test will go a long way to satisfying your obligations in operating your business within the law. A PAT test is also often a condition of insurance for both you and the venue - this is arguably the biggest issue for venues and musicians and why venues will insist your gear is PAT tested.
A PAT test, rather like an MOT on a car is only a safety test on the day of the test. Obviously, you can tread on a lead or drop an amp when you get to the gig and your equipment may no longer be safe. Good handling and safety monitoring is also necessary. We can advise you, at various levels, how best to keep safe and stay within the law.