Yes you have to pay 21 pence per mile on top of your hourly/daily rental. There is a petrol card inside the car if you need to refill (You can earn tesco/nectar points at the right petrol stations.)
Except it says "NHS Staff/Volunteers, Emergency Services, Armed Forces & Key Workers"
From the page you linked "...all members prior to renting are to ensure they:(...)Are not living with or in close contact with someone who is currently quarantined or who has tested positive for the virus."So looks like they're not eligible anyway?
Do you have to pay the per mile rate on top of the hourly/daily rental fee or do you choose which of the tariffs you want to use?