Emergency credit charges

I had to use my emergency credit yesterday to get £5, I wasn’t able to top up until today when I got home from work. My meter was showing £1.79 so I went to the shop and topped up £20, I’m a bit shocked at how much is now on my meter after topping up, I’m left with less than £8. I do understand that there is a charge for using the emergency credit but that seems a bit much to me. Can anyone help me with this? Thanks