The last Train Manager I spoke to said that the lighting wasn't alterable. It may just have been unalterable by GWR▸ staff but possible if you're Hitachi. I might have a look through the DaFT» specifications and see if it was a requirement or not.
The
IEP▸ specification does not itself dictate general cabin lighting, but calls up BS EN 13272:2001 ‘Railway applications – Electrical lighting for rolling stock in public transport systems’ (superseded as :2012) as it applies to main line rolling stock. You'll do well do find a copy of this, as
BSI▸ (and I suspect other standards institutions) have taken to using DMCA complaints to Google to suppress on-line texts.
There is a little about emergency lighting, including the only number (20 lux - not at all bright), and one mention of light levels at seats:
TS1511 At each seat position, the following features must be accommodated. Each of these features must be capable of being incorporated or not as required for each deployment of IEP Trains (different features may be incorporated for different classes in the same deployment);
...
• sufficient lighting level to be allow reading when seated
...
Reads oddly, doesn't it?
The internal fittings and features lists do include reading lights and table lamps, but neither is ever required even in first class.