Guys I have the same phone K770 with Virgin UK. I need to debrand it as well.
I have got the Main and the FS, and the custpack for the phone. But I am confused regarding the custpack. The files that I have are:-

K770_R8BC004_MAIN_GENERIC_VI_RED53
K770_R8BC004_FS_EUROPE_2_RED53

custpack
HB1-06_Gen_CDA102568_11_R8A_GENERIC_EUROPE_2
K770i_1202-3495_R8A_Virgin_United_Kingdom

Which custpack I should use the first or the second?

I hope you'll help me as well. Thank You