Why we Did Not avail of the 20%discount at Nishimura?

We saw this 20% discount. We’ve read it but it was not a 20% discount automatically to your bill. You need to spend £50 to avail of the 20% discount. Our bill was more than £50 and we were told that since it was £_ _.00, we need to top up to £100 to avail of the 20% discount. We think it’s ridiculous, our bill was more than £50 yet we can’t avail of the 20% discount. We need to pay more to reach £100 to get the discount. The remaining money will be stored in your Nishimura card for future consumption.
This was the reason why we did not top up and was not able to avail of the discount.
Food review:
Sushi/sashimi tastes nice.
The beef in the kimchi ramen soup is a bit chewy.