Default [Q] Custom ROM on Xperia Sola Bootloader lock

Hi,

It's the first time I try to change my Rom on a phone.
My phone is a Sony Xperia Sola Rooted (with Kingo Root) and I want to install a custom Rom.

The problem is my bootloader is lock and it's impossible with the official Sony process. (I have Bootloader unlock allowed : no)

How can I install a Custom Rom with my Bootloader lock ?

Thanks for your answer.