Thanks for your info. Your link to “Andrid Device Compatibility” really seems to be a good help, because its given me another angle to explore.
I will keep documenting what I find for anyone else going through this process. 
That page you linked to talks about “reference feature tiers”, so at first I was wondering if there was a slider option for feature tier? i.e. these tiers:
LDR
Basic Lighting
Full HDR
Full HDR w/ Sun
… but the more I look for them, the more I think they are fragmented up across a number of different editor areas and settings options?.
I’ve found so far two main areas of the editor which effect what I think are tier like options?
In UE4, the more obvious (in hindsight) page I found on the Project Settings Rendering page, i.e. (EditMenu/ProjectSettings/Engine/Rendering).
For my new cooked version, I’m going to try these settings:
Mobile HDR (Off)
Occlusion Culling (Off)
Texture streaming (Off)
Allow Static Lighting (Off)
Tessellation can’t be turned off (but I assume theres no attempt to do software only Tessellation on the GLES version).
Separate Translucency (off)
I also found a useful Editor option on this rendering page to change Editor MSAA. Turning it off helped on my lowly laptop. … (In my editor, I now have up to 8 FPS!)
The 2nd less obvious area I found on a submenu to the big Build icon button above the main 3D window. There’s a lighting quality setting submenu, and that’s set to Preview. Hopefully with that on Preview and Static Lighting off, UE4 won’t attempt to bake any lighting and so help to speed up
the cooking process?. I’m going to cook a clean version of the project, to force a full rebuild, to see how long it takes?
I think I’m going to give ETC1 a try like you say. I was put off from trying cooking just for ETC1 before by the long cooking time, until I was certain what to do for the next version, but it makes sense to use some compression if possible. (By the way, for anyone interested, I found ETC1 support was added to Android version 2.2 (Froyo). Also the Galaxy S1 uses a PowerVR SGX 540 and I see from the Android logs coming out of the Unreal App that I’ve created, Unreal does detect it correctly, which is some good news.
I’m now cooking another version with these settings above. (While it cooks again, it’ll giving me time to write up where I’m up to!)
I’ll let you all know how it goes?..