Is it permissible to buy and use a railcard that costs £20 and offers a one-third discount on ticket prices for a year? If it is not permissible, should the card be destroyed, or can it be used until its expiry date?

It is impermissible to use discount cards that are not free, due to the element of gharar (excessive uncertainty) involved; the buyer pays money for a potential and unknown benefit. What was dealt with before knowing of the prohibition is forgiven.