I honestly don't think there is much difference in the appropriateness of the food diabetic versus "normal", for anyone low carbing. I took particular note to really look on the ocassions I did order the diabetic option. Whilst it's not so extreme in Premium, you do receive your meal before any companions you might have, so will either be leaving your meal to cool, or eating in advance of them; which feels uncomfortable to me. Bearing in mind the portion size of any pasta or potato, it doesn't feel like a mega hazard for me.
I must admit, I did make sure I had a couple of small packs of salted peanuts in my handbag, so that if it all went horribly wrong, at least I had something I could go to immediately, without making a bit fuss. As you say, it's only a few hours, and on the way back, you'll probably be asleep for much of it.
(And there's always the M&S Food as you come out of Arrivals at Gatters, if you happen to end up starving. They'll have yoghurt, cooked chicken and so on.)
Enjoy your trip.