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The ignorance is strong in this one.British police search and rescue divers spend extended periods at over 40m using just air, with just a single 12L of air on their back in case the umbilical supply fails. This fact seems to contradict the requirement for helium in the mix or having a large amount of back up gas for decompression.
If the umbilical fails, that diver will most probably switch to their bailout and then start their ascent immediately. Now, even owing a noticeable amount of deco, it's unlikely that they'll need to spend a significant amount of time at extended depth. How long would a full 12L (I'm assuming 232 bar here) last for ascent and deco? Hint: for quite some time. Also, since they're diving with an umbilical and a surface tender, it's unlikely that they have to include their non-existent buddy in their gas planning. Also, since these blokes are rather well trained and likely spend more time under water during a month than you or I do in a couple of years, they'll probably use less than the 2x gas consumption normal sports divers ought to reckon with if something untoward happens.
I don't know the standard ascent speed for British tables, but I wouldn't be surprised if it's rather close to the 10 m/min they use in our tables. So even if they don't have He in their bailout tank, they'll probably be at depths where narcosis isn't particularly noticeable in a rather short time.
How much experience do you have with diving deep-ish on air or lean nitrox? I know that people are very different when it comes to how much they notice narcosis, but based on personal experience I'm not very comfortable with going much deeper that some 30-ish meters on air or nitrox. Both because of narcosis and because the gas density is getting uncomfortably close to what is now considered problematic somewhere between 30 and 40m. Gas density affects CO2 ventilation, and CO2 is considered extremely narcotic in addition to its other rather unsavory properties.
I have a strong suspicion that I won't get through to you either, so I'm writing this more for the benefit of others who might stumble over this thread.