To top up something

Both proposed formulations are ambiguous. They make it unclear whether (1) the amount of the addition is supposed to be 10 pounds (regardless of what is already in the account), or (2) the total in the account, after the addition, is supposed to be 10 pounds (i.e. one wants to add whatever amount will bring the total to 10 pounds). The clearest way of saying (1) is 'I'd like to add 10 pounds'; if one wants (2) one can say 'I'd like to top up to 10 pounds' or if one wants to completely remove any possibility of misunderstanding ''I'd like to bring the balance up to 10 pounds'.

The phrase top up carries the idea of some limit, top, that is to be reached, an therefore fits only (2); it would be confusing to use it if (1) is what one intends. But if (2) is what one intends, then the preposition that should be used before the limit is to, not for.