If you didn’t drive and needed taking somewhere (20mile round trip for the driver) and then collecting later, so another 20 miles, would you think the driver was unreasonable to ask for some petrol money?
If a taxi was going to cost them e.g. £30, would it be unreasonable to ask for £10?
For context, the person in question is a younger family member who doesn’t drive (their choice) and I’m being asked to drive them. It isn’t a ‘oh you’re heading to x, can you drop me off?’ It’s a ‘I need to be at this place at this time and picked up at this time and need you to drive me.’ No offer of petrol money.