Practicalities aside, really this whole topic is a debate around whether or not you believe USB charge points (and would appear now wifi also), are necessary or not. My personal take is that they absolutely are - aside from being something I often use myself, in an increasingly digital world, it's something the population en masse are coming to expect as most public transport offers such facilities. The issue with the "take a charge pack" arguement is many people (myself included) won't, as it isn't advertised anywhere that these trains dont have charge points. I would suggest the average member of the public is sympathetic enough if their expectations are set prior to boarding, but when something that is so commonplace and so basic is absent, it really becomes a glaring omission.
Any half decent phone, tablet or laptop has more than enough battery life to make charging part way through the day the exception to the rule, and an awful lot of people now have and use power banks (of course you need to remember to charge them too!). Many devices offer reverse charging, so you could bung a bit of power into your laptop from a phone or vice versa.
Apple now has a reserve power feature for travel, which is simple but clever and maybe more will follow with this idea too.
I really do think power on a train is a 'nice to have' and not absolutely essential, although I will agree that people do perhaps expect it now (even if they don't often use it). Just like Wi-Fi, which is often not the best choice for reasons I've mentioned before.
O2 data plans are very poor, then. I have to add more data as a bolt-on and it gets so pricey, that I need Wi-Fi.
O2 has loads of plans, so I suggest you look at changing if you're having to always add more data before the next month. If you are on prepay then perhaps unlimited data isn't an option, but if you're needing data then why not look at a 30 day or 12 month SIM only plan?