ONCECIMBER,
You should in theory be able to work in BA with an unrestricted JAA ATPL NOT necessarily issued by the UK CAA . The burning question is whether BA will hire pilots with licences issued by other JAA countries and not the UK. With the present flight ops management and specifically the man in charge ( PoD for those wondering who he is) I do not believe this will happen. I hope I am wrong and wish you luck.
Best regards
The Viking