Parcelforce48 will allow it as they garuntee delivery over a set time period. The reason you cannot just post any old gas at any postage rate is because a package could be sitting in a depot over a weekend that isn't manned and therefore could otentially go pop and burn down some shit. With Pf48 the item is marked for express courier delivery. It's a decent price too.
It's probably not worth it, tbh.
The cost of sending them will likely be more than the can is worth anyway.