I tried working it out once and there were so many noughts involved it did not matter.
I have a vague memory of around 10,000 times the volume of gas per £ in BOC bottles opposed to the little disposables.
Try to get an account with anybody but BOC if you can, a load less hassle and usually cheaper.
see if you can find an independent supplier, BOC used to have almost a monopoly over gas supply, they act as though they still do. Go in yellow pages and see what you can find, google energas and industrial gases.
you almost defiantly won't use a whole Y to restore a golf, I used less than half an X to replace the sills and rear floor in a land rover discovery, thats with other jobs in between too, though I don't you mig for much other than cars, mostly used oxy acetylene for making stuff, (I know I know, more expensive, slower, and more distortion) heh tig now anyway