We have zero doubt that by the end of your first 6–12 months, you'll be earning above £120k. Not because we're handing it to you, but because of what we've seen from you through the process and what we believe you can do here.
As a company we need to see evidence of it before we pay according to it. The £105k today is the floor, not the ceiling.
The six-month review is the first formal step. Hit what we've agreed together, and your base moves.