You may be able to acquire a pub CO2 bottle from a friendly landlord (do you have a local where you spend a bit of wedge??)
CO2 can produce a pretty hot weld pool though, so is a little bit harder on thin steel, like the 1.6 box. Argoshield Light (or Cougar) is nice to use.
There is a firm that does rent free 9l bottles, just pay £40 deposit, but you might get hit for the delivery charges -
adams gas because they are in Kent, although they use TNT to deliver them.
Now, I get lost outside of the M25, but these 2 'might' be local to you, although I have no idea what they offer/prices etc.
The details are on the 'gas suppliers' thread from the mig welding forum
here
Energas Ltd
Salmon Pastures, Attercliffe Rd, Sheffield, South Yorkshire S4 7WZ
Tel: 0114 273 1730
TC Welding Supplies Ltd
The Chapel, Carrwell Lane, Wadsley Bridge, Sheffield, South Yorkshire S6 1NR
Tel: 0114 234 1149
Hope those help