Many Carmina lasts are accommodating for low arches and a low instep, but it also depends on what style you are looking for (i.e oxford or derby) as the same last might fit differently based on style and material. I would recommend wearing the same size in Alcudia, Simpson, Robert, Queens, and the loafer adapted Uetam last as you wear in Edward Green. They are all notably snug around the instep. Oscar is also somewhat snug but is more roomy in the vamp and the toe box than the aforementioned lasts.
Thank you. Simpson in size 8 wouldn't be too small, i.e. short and/or tight around the toes?