So one thing that's been going through my head.

Is there any reason I couldn't use BSP pipe as opposed to steel conduit? it's made to a BS standard after all....

I ask as the BS standard for conduit appears to have specifications for 38 and 50mm but nobody stocks them. However, 1 1/4 BSP is easy to get hold of and is thicker walled due to the courser thread pitch. So I don't see why it couldn't be used. I'll have a single bend so putting in a draw string will be easy, and a long radius bend will be well above the minimum radius I'll need to make the corner at the top of the vertical.

1 Gas pipe

Someone will pop up and say that black or galvenised  iron gas pipe is made to the wrong BS. of course.
(This stuff.. )

That is true but not  really relevant, there  is plenty of imperial conduits about and no one asks about that. The hard part may be  getting a good earth contact to a malleble iron pipe of non-standard diameter, but it should be possible to mis-use a plumbing fitting or to drill a hole for a nut and bolt  (or even tapped hole )  in a way that does not obstruct the cables.

https://www.bes.co.uk/flanged-hexagon-backnut-brass-11-2-6549/  going into a box perhaps

The exact impedance of  gas pipe may be unclear so you may want an additioinal CPC to assuage the doubters.
The other thing you will need are the right pipe threading dies and a decent pipe bender, but those could be  hire items unless you know a tame gas fitter type. And bending a 1.1/2 iron pipe will test your muscles  and may benfit from hydraulic assistance.

2 SWA - ideas

Or if it helps with SWA, remember one can terminate the armour in a gland on a metal box or plate in the ceiling and carry the inner as a rather wilful multi core "flex" into a further enclousure.
Arguable the inner 'bedding' that holds the cores together is not really insulation, and so the "peeled" SWA should be inside something like plastic trunking, rather than run any distance exposed to touch,  but it is a lot tougher than broken out into singles straight away. Leaving very  long tails allows them to be led into the CU in a less ugly way for termination.

M

1 Gas pipe

Someone will pop up and say that black or galvenised  iron gas pipe is made to the wrong BS. of course.
(This stuff.. )

That is true but not  really relevant, there  is plenty of imperial conduits about and no one asks about that. The hard part may be  getting a good earth contact to a malleble iron pipe of non-standard diameter, but it should be possible to mis-use a plumbing fitting or to drill a hole for a nut and bolt  (or even tapped hole )  in a way that does not obstruct the cables.

https://www.bes.co.uk/flanged-hexagon-backnut-brass-11-2-6549/  going into a box perhaps

The exact impedance of  gas pipe may be unclear so you may want an additioinal CPC to assuage the doubters.
The other thing you will need are the right pipe threading dies and a decent pipe bender, but those could be  hire items unless you know a tame gas fitter type. And bending a 1.1/2 iron pipe will test your muscles  and may benfit from hydraulic assistance.

2 SWA - ideas

Or if it helps with SWA, remember one can terminate the armour in a gland on a metal box or plate in the ceiling and carry the inner as a rather wilful multi core "flex" into a further enclousure.
Arguable the inner 'bedding' that holds the cores together is not really insulation, and so the "peeled" SWA should be inside something like plastic trunking, rather than run any distance exposed to touch,  but it is a lot tougher than broken out into singles straight away. Leaving very  long tails allows them to be led into the CU in a less ugly way for termination.

M