Portable cylinders for 300bar usually top out at 12 litres. Down here in rural Devon we face some of the same issues that you do, and the round trip to our most favoured fill station is about 140 miles. There are other places somewhat closer but each has its own disadvantages such as inconvenient opening times, nowhere to park, incomplete fills, and so on.
For us it makes sense to gather together several bottles and make a pilgrimage to the dive shop. Normally we take at least four 12 litre bottles and sometimes up to a dozen. It takes the dive shop a while to fill 12 bottles so we find a place for lunch and pick them up a couple of hours later. Makes a proper day out.