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FGW have 54 HST sets, plus spares. So around 450 coaches. With an average of 40 sockets in a coach (one per every other seat in Standard, slightly more in First), that's around 18,000 sockets to maintain.
I imagine the sockets are rarely used? On a commuter train, I can't see many people needing or wanting to use the sockets (unless they're desperate) as I am sure that in most cases, a smartphone or tablet can last long enough until it can be charged more conveniently at home or in the office.
And I don't see that many people wanting to mess around with a laptop charger on a train sitting in an airline seat when people are getting on and off every few minutes - and if you don't have an Apple-style charger (magnetic) it's even more of a faff as things get caught up. Far easier on an Intercity train where the next stop might be 45 minutes away and there's usually a bit more room too.
That's just my opinion but I don't see much need for sockets on commuter trains, but can see why a TOC might see that this is a way to actually offer something more to those paying for first class - considering commuter trains usually offer very little besides the higher chance of getting a seat.