Don't know how LPG supply works in UK. In NZ we've few different bottle sizes with 9kg being most common by far. Most garages offer swap a 9kg bottle instead of filling. LPG company supplies garage with full bottles and takes the old ones away for filling and testing if needed. Works out about twice as expensive as fill but very convenient for both customers and retails which don't need LPG infrastructure. Swap a bottles are available in most small towns.
Price is same regardless of how much gas is left in bottle, unless customers have 2 bottles they rarely run one dry before swapping.
Can still fill bottles and need to if using 4kg and 14kg, but fill stations are far and few between, maybe one or two in a city.