Default [Q] Xperia L - help in flashing/updating

Hi I would like to ask for your help on working with my Xperial L. It is free phone from my network plan so when I checked the unlock bootloader is set to NO. that is my problem at the moment. Is it still possible to root, and port a kernel/custom rom even if the bootloader cannot be unlocked? I hope you could give me the proper steps so I can arrive at the same set up as yours.

Here are the details of my unit at the moment:

Model - Xperial L C2105
Android version - 4.1.2
Build number - 15.0.A.2.17

Bootloader unlock: not allowed

I haven't started anything yet as I'm unsure which to steps to follow, hope if there's someone who had the same details can help me to root and port a kernel/rom.

Many Thanks in Advance!