It's actually reasonable suspicion, unless under the terrorism act.... but a broken tail light is not reasonable suspicion for stop/search powers but is a very reasonable reason to stop a vehicle.
Always ask for the grounds, and always say if you're not happy, but remember that force can be used to conduct a legal search. If you don't think it's legal, complain afterwards. If you fight against them and get arrested then regardless of whether or not you agree with it, if the search was justifiable (in law) then you'll end up with chartges of Asaault on Police.