Call your insurance co and ask them if your policy "covers the contents of your garage". Don't tell them what the contents of your garage are, just ask them that exact question and get a straight answer.
N.B. they will no doubt state that vehicles are not covered, as you have separate insurance for vehicles.
However, you will probably find that the contents of your garage are in fact covered, so long as the garage was locked. It doesn't matter that it was clothes that were stolen. Theft is theft, and there is no law that says you have to keep your clothes in the house, and it's unlikely that your policy will say otherwise.
It's up to them to prove that you were given sufficient information when you bought the policy that clothing stored in the garage was not part of the cover agreement.
Keep records of all your contact with them, don't divulge any info you don't need to, threaten to complain if you don't get anywhere, and finally go to the ombudsman if you're not happy with their final decision.