You can contact an agent and they will apply some credit to your account to take you over £20.
Did you top up with a voucher or a card. Topping up credit is very risky as you have found out to your cost. It's best practice to always switch data off before you are buying a goodybag...or buy the goodybag in advance, long before the old one runs out.
If all you want to do is exchange the credit for a goodybag then you might consider buying the goodybag directly. That way your credit won't get used before you have applied the goodybag