Hi guys, I have a quick question. I bought my iPhone 6S in the country where we don't have an Apple Store (imported from the UK). I'm not happy with it as the backlighting is very uneven (top part of the screen a lot darker). Still using my iPhone 6. Anyway, I'll be traveling to London soon. Can I just walk in and ask them to replace my 6S with the new one or I must create Genius Bar appointment before coming?