I don't know about the "gas bottles" (I assume that they are similar to propane bottles over here) in the UK, and what type of valve they have, but I am 99% certain that the OPD valves required in our bottles will not vent just because you open the valve, upside down or not. They have to have the male portion of the hose inserted before any remaining propane will exit. MAYBE if you removed the entire valve first. So, there is no way I want to start cutting holes in one...
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OBG