There are over 1200 filling stations around the country, In Accy, Dill Hall and the Shell at Rising bridge do it(Nightingales used to not sure if they still do as I rarely get that way). The cheapest places are usually Calor bottled gas outlets or even some Farm shops do it ( I use one just off J19 M6) .
I've never been short of somewhere to fill up and even if I was I can switch back to Petrol as and when I choose even whilst driving along.
This is my 4th LPG car now. I've converted the previous 3 but this one was already done. In all cases I 'saved' well over double the cost of the system. My last car ( Volvo) covered nearly 100K miles on the LPG system.
In my case as a Petrolhead it allows me to drive cars i want to even though I do high mileage. Without it as an option I'd be stuck in a smelly, noisy DERV.
I've covered ~ 250K miles on gas now so as you would guess, I'm a convert.