I don't really have a standardised driving pattern or fill up a certain time / day / amount, so I don't really know, and I think it varies too much, and I'm scared to know the answer...
I drive different amounts each week, and fill up a random amount depending on how I'm feeling, more so if I accidentally go over getting a perfect multiple of £5 or £10 and have to go to the next one until I do so.
Though as a rough guess, 90 miles per week if I'm not going anywhere extra, 28mpg, Tesco Momentum at 122.9 per litre...
Soooooo.. about £18 per week?
But that's probably closer to £60 if I have to travel to Newcastle over the weekend.