Why such a big step to technical diving?

[chrisch]

I certainly am not about to do anything stupid. Regarding PO2 and decompression obviously a good understanding of these is essential before exceeding normal NDLs. Post #15 asked my current quals. I am qualified to dive to 40m amd nitrox qualified. My own kit is single 15 L tank. I am basically a warmer water holiday diver. I could go to my local quarry and do loads of <20 m dives just to get my numbers up, but other than doing a few to keep in practice that seems pointless. I am a member of a BSAC club and quite a few of them use rebreathers. For the relatively small number of extended deeper dives I would want to do, a rebreather does not seem to be worth it. The expense of an extra cylinder and regs is small by comparison and additionally the maintenance requirements are much less / simpler. Bearing in mind the high "per minute deep bottom time" cost of holiday diving a twin tank 2 gas setup be the simplest way of increasing deep bottom time and extending it a bit over 40m may be to use a bottom gas, may be air or something with less nitrogen and use a high O2 % nitrox for the shallower / accelerated decompression parts of the dive.
My previous dives to 30m + seem all too short, hardly enough time to relax and get a good look round a wreck.

The deeper you go the more nitrogen you absorb. If you use another gas like helium you will absorb more of it too. For this reason all diving past 30m is either a short bottom time or a long decompression time. A rebreather is the best way that exists to maximise bottom and minimise decompression. If you cannot justify the expense and training time (I cannot either) then you are back at square one and must either have a short dive or a long hang.

You can speed up your decompression with a higher oxygen percentage. The BSAC course is Sports Mixed Gas. The IANTD course is Advanced Recreational Trimix. Both of these give a better bottom gas and a faster deco.

For diving in the 30-40m range this is IMHO the best way to go if you don't justify the rebreather option. Having a mixture of back gasses is generally thought to be a stupid idea and you will look long and hard to find anyone doing it. Your current equipment level and training are not sufficient for extending the 30m+ beyond a few minutes and I totally agree this is not enough bottom time for anything like a good dive. I tend to work on about 40-70 minutes bottom time as a "decent" dive.

I think you will struggle to find the BSAC SMG course but the TDI and IANTD courses are widely available in the UK. Have a look at what is required and think about the cost-benefit to doing the training. In my experience BSAC clubs are not the place to find very good information on advancing your diving in this area. If you get the chance do a GUE experience day, they run at Capernwray sometimes. It's a lot of effort to get that extra bottom time but I agree with you if you look at the cost per minute underwater the return on your investment is worthwhile.

 

[Miyaru]

@60plus
As mentioned by others, the big step is not the equipment, but the mindset.

Until now, you have received training to dive safely with the mindset that everything goes right. A few exceptions are out of air, where you rely on your buddy, or a runaway inflator, which can be disconnected. In all of these situations, the dive can be ended by a direct ascent to the surface. Reaching 50bar is the mark where you end the dive and ascend.

Diving doubles in a dry suit doesn't change that. The same recreational rules still apply.

In technical diving, the mindset switches to everything might go wrong, and having a contingency for each of these situations. There is a whole world of posibilities (and dangers) beyond the recreational depth limit and the no-decompression limit. Recognizing, preventing and handling those dangers is what a recreational course does not provide and technical training does.
Diving doubles with nitrox extends your dive. If that allows you to reach your goal, do so. If reaching your goal requires crossing limits, take the next step.