Is Zakat obligatory on a sum of 3,500 British Pounds, and if it is, when should the giver pay it and what is the obligatory amount, keeping in mind that he already gave a portion of the money with the intention of Zakat last September?

A quarter of a tenth (2.5%) of the amount that has reached the nisab (definition) and on which a full year (hawl) has passed must be paid. The nisab is the equivalent of 85 grams of gold or 595 grams of silver. A quarter of a tenth is calculated by dividing the amount by forty. If what you paid in zakat (definition) is equal to a quarter of a tenth, then you owe nothing further. If it is less, then complete it. If a full year has not passed, then zakat is not obligatory, but it is permissible to pay it in advance.