I'll explain to you.
On a Ltd company, you will pay corporation tax on profits @19%
On a self employed income, you will pay income tax on 20% or 40%. Given the fact you already have a job with the NHS, i am expecting you would end up paying a higher rate of tax either 40% or 45%
Furthermore, you will also pay class 2 at £3.15 per week
and class 4 nic 0.25% on profits above £11,908