It’s hilarious to me how upset some people get over this 
I think it’s more jarring for native English speakers since they have more exposure to only one of the spellings, while for me it’s more of an even split how much I see them.
Especially the “-or” / “-our” endings are common enough that I’m very used to them (and might actually use both spellings myself depending on mood). The spelling “centre” looks more unfamiliar to me, I think because it’s less phonetical, and I’ve never used that spelling myself.
Edit: I should clarify that I think it’s very understandable to be frustrated that a .co.uk domain uses American spelling without stating that in the rules, it’s just that my reaction would be “ah ok, now I know that for the future” (kind of how my reaction was when I found out that duplicate letters are allowed and there are some hidden rules for how they are marked) rather than “UNPLAYABLE!” 