Not sure what version of MMS you are using but we are aware about the issue for
Symantec Mobile Management 7.1 SP1 and earlier. Do you have it under MMS 7.1 MR1 or MMS 7.1 SP1?
The British English internationalisation uses a different language code than regular English.
Currently, there is no way to get a British English device to view the Mobile Library for English. The only way to access the Mobile Library is to switch the International setting on the device to run in English mode.