Shower waste drains and regulations.

I will shortly be installing a shower cubicle in my bathroom. I cannot place the drain below floor level (because the joists run the 'wrong' way!', and so will have to use a raised 'Easy Plumb' shower tray which should give me the necessary 'fall' to the soil stack which is in the opposite corner of the room to the shower.
To gain the maximum 'fall' possible I want to use a shower waste trap which places the drain pipe as close as possible to the underside of the tray and hence as high as possible off the floor at the start of the run. The problem is that most shower waste traps have a shallow 'seal' and, where the drain goes directly into the soil stack, I understand that this is not acceptable and there has to be a deeper seal ( I have been variously quoted 50 mm and 75 mm - anyone know what the true figure is?).
There are a couple of ways around this problem - I could fit the shallow waste trap to the shower tray - but also install a deeper 'running waste trap' somewhere along the run - most obviously where the pipe runs behind the bath panel. I can cut out a section of floor board and drop the 'U' of the running waste trap into the void.
Another alternative is to use a HepVo trap - either on its own, or in addition to the shallow waste trap. Does anyone know if the 'membrane seal' design of the HepVo is acceptable under Building Regs as an alternative to a deep seal on a conventional waste trap?
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