Minimum charge is a good idea and £10 a good minimum charge.
But business guidelines should be just that - not inflexible.
So if I did a group of semi's at £10 each and someone in the middle who was very security conscious said fronts only please I would happily charge say £7 for the front only.
But I would let them know I was making a special concession for them.
If I had to reel in my hose and move my van too far for such a customer then the minimum charge would apply.