Whilst I too sympathasise with the OP, there are a number of reasons that people wish to dim the tint on the windows, indeed at altitude, there is of course increased ionised radiation and perhaps they want to utilise the ALARP principle contained in international convention. Many can attest despite your experience on your return route, direct sunlight does degrade the quality of IFE, this is dependent on the angle to the sun.
As someone who has been and studies the effects of only having artificial light for months at a time; you are unlikely to suffer on an 11 hour flight of Vitamin D deficiency. The CC clearly had sufficient light to be able ot serve the food, and with the personal lighting available in your seat, I can't see anyone realistically having trouble seeing their food whilst illuminated. It might be that the lack of natural daylight was not the cause of your lack of sleep upon arrival, but I can recommend wearing orange tinted glasses before sleep if you do have trouble, since they block blue light .
Overall, it is a timely reminder for us to consider our actions on those around us, but that goes to the OP demanding to have their preference of lighting over anyone else as much as it does to anyone sat in the window seat, who may have an equally valid reason to dim the window. As for talk of it not being their window, it being BAs, people pay a premium to select window seats and BA gives them the operation of the window blind as a privilege.