If you find a place let me know. I use BOC in Northampton, but the rental is a bit of a sting.
Adamsgas is the only one I can think of who will do it by post. Would make sense if your usage is low. Otherwise a CO2 bottle from a local pub is the only cheaper option I'm aware of.
CO2 isn't ideal - the arc is less stable so it's trickier to use the welder to the maximum of it's adjustment (which you need to on thin stuff) and it's a bit hotter apparently. Lot of people put up with that, though I'm not one of them as I like things to be easy.
Thanks for getting back. That's not a lot better than where I am with BOC (I go for the Y size, and I think you've got W size which is twice as big). Rental is always the killer with BOC and AP. My rental is less, but my bottles are more to refill.