regardless of the actual system locale addlocale will always show "en_US" because this is the default for the system reset. I don't understand why you get the error message, you shouldn't, and this usually indicates either an incomplete addlocale run, or running addlocale on a borked system. Do you get these messages also when you open a terminal? On the other hand, if all works fine, then you could just ignore these boot-up messages.
But what exactly means
addlocale seemed to install ok? Seems to me that you maybe started again from an incomplete addlocale run? Ibiblio seems to work fine again now, so please ensure to
1) have a functional Synaptic setup, see
1, and/or
2, then
and only with a functional Synaptic setup:
2) run addlocale and reset to en_US (in order to restore the default system setup)
3) run addlocale and choose en_GB
this should work fine and not provide any error messages.
regards,
-p.