I've gone for an Off-Peak single down and a First Class Off-Peak single up. I couldn't opt for an Advance on the way back because I've no idea which train I'm going to want to get, and it was still more expensive than cattle-class on the way down (maybe Weekend First isn't a thing on London Trains, I don't know - Weekend First is where you get a first class ticket for only £15 extra). First class in the London trains often has a decent amount of free food (which on other trains it doesn't) and it definitely has more space (including single seats on one side of the carriage) and usually you can send the attendant to buy food for you from the shop rather than having to trek down the train yourself (which I think I will definitely appreciate).

I'm not 100% sure B really knows what he's talking about wrt. icing - on the other hand totally not worth debating when I could be in the bath.