This can be a very good alternative for short term taxi insurance as most drivers only work for 4-5 days only and they would consider if there will be a saving.
If the prices can be reduced competitive to annual / monthly insurance which approximately cost £200-250 a month,
My suggestion would be
1 day £15
2 days £20
3 days £30
4 days £40
7 days £65 ( this will make you to compete with other annual insurance providers )