There are two ways to answer this question. On the one hand, you can say that the British in general wanted to seize these weapons. The weapons were being stockpiled so that they could be used in the event that the colonists’ rebellion ended up in violence. The British, naturally, did not want their people to be able to conduct an armed rebellion so they wanted to seize the weapons.
On the other hand, if you need to give the name of one particular person who wanted to seize the weapons, you could give the name of the leader of the British in Boston. At that point, the leader of the British was named General Thomas Gage. He was the governor of Massachusetts. He needed to show the colonists that he and the government in general were serious about cracking down on the rebellion. He thought that seizing the weapons would be a good way to do so.
So, we can either say that Gage wanted to seize the weapons or that the British, in general, wanted to do so.