All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If the currency is still in circulation, then the rulings of the sale of currencies apply to it. It is not permissible to sell a currency for a greater value of the same currency. So it is not permissible to sell £5 for more than £5.
The fact that the banknote has special numbers has no effect on the ruling of exchange, and this must be taken into account.
However, if the £5 note is sold for a different currency, such as selling it for US dollars, for example, or other currencies, then there is nothing wrong with this.
It should be noted that the acquisition of these currencies just because of their numbers is a waste of money and a misappropriation of money, and a person is responsible for his money, from where he has acquired it and where he has spent it.
For more benefit on the terms of currency exchange whether they are of the same kind or different, please refer to fatwa 88034.
Allah knows best.