And even if all people know of Superman is from the movies, did any of them actually listen during the Krypton scenes of the first Superman film? Everyone was British, for heaven's sake - even Brando put on an accent. Susannah York played his mom! Zod was Terrance Stamp. Sarah Douglas was Ursa. When they made Supergirl all the Argo City people were Brits. By rights Christopher Reeve, Helen Slater and Brandon Routh could have played their characters with British accents and it would have been totally correct.
Slater, yeah, since her character was raised by Kryptonians. But since Clark Kent was raised from infancy by a Kansas family, and presumably learned to speak from them, your argument makes no sense for him. Accents aren't genetic. (Although of course it makes even less sense that Kryptonians would speak English at all, let alone with an English accent.)
I don't like any of the suggestions I've heard for Lois. People are picking glamour babes, and that's not the most important factor in casting Lois Lane (as the example of the achingly gorgeous but totally miscast Kate Bosworth should make clear). Lois should be cast for personality first, that hard-driving, fast-talking, Rosalind Russell/Kate Hepburn type. Erica Durance nails it. Dana Delany nailed it. Margot Kidder did it pretty well. Teri Hatcher kinda did when she wasn't being all weepy and neurotic.
The casting pool for Lois should ideally lean more heavily toward comediennes than glamour babes. Actresses who have sass and energy and attitude, who know how to put expression and intensity into their voices.