Hello.
Declaring the tax & N.I. is not enough on it's own, per se, to show you are self employed. What really defines you as self employed, from a mortgage point of view, is your proof of income.
So, as a very general rule of thumb, if you have payslips/wageslips, then you are employed. However, if not, it's likely that you'll need to show your income by accounts from your accountant or tax assessments (which I'm assuming you don't have).
If you've 'just started' it will be difficult to get a mortgage without a large deposit (25%) otherwise proof of your income will be essential. Alternatively, a larger deposit may allow you to self certifiy your income but the interest rates tend to be higher.
All the best.