What I dont understand is why I can afford a good £200 more a month mortgage repayment on what I currently pay, but I cant get a mortgage for the increased amount

Is that a genuine reason to get a self cert mortgage? my overtime varies massively so do I have a genuine claim of not knowing my exact income?

I dont want to borrow massively over my budget and can easily afford the £200 extra, I already know my cost of living so I've already worked out my budget and the only thing holding me back is how much I can borrow