Here's a few assumptions to help you:
Cost of fuel = £1.40 per litre = £6.3644 per gallon (1.4 x 4.546)
If we assume 20 mpg, then you're using 4 gallons, so cost of journey for fuel = 4 x 6.3644 = £25.46
If we assume 30mpg, then you're using 2 2/3 gallons, so cost of journey for fuel = 2 2/3 x 6.3644 = £16.97
If you give him £30, that works out at roughly 17 mpg ......
All of the above is just for fuel only and takes no account of any possible toll charges, wear and tear of the car, etc ..........