Over the years there have been quite a few posts about giffgaff's use of terms such as "Nice One", "High Five", etc. It seems to be a bit of a Marmite issue, you either love it or hate it. I suspect many people, like me, find it childish and banal, but most can't be bothered to comment.
So why not introduce an account setting that allows users to choose what kind of "personality" they would like? I don't mind how "creative" you wan to be as long as there's a "businesslike" option.