In my experience, it's a lot harder to go a size up on derbies than boots. I've tried doing so, and if they don't fit right in the length department, my heel will still slip. I usually size my boots up a size to a half size to accommodate an insole but I can't do the same with derbies. Your results may vary, however. I believe Farfetch has free returns - might be worth giving it a try?