Freelance Work

As a very rough rule of thumb (assuming you aren't working through a company), you pay income tax on your entire income for the year. So if you add up your annual income from your part-time work and your annual profit from self-employment and put that figure as your annual salary into an online tax calculator, that will give you a rough guide to how much tax you will have to pay in total, and then you can compare that to the tax from your part-time work on its own to see how much extra you will pay. It won't be exactly right, because NI is different for your employed and self-employed work, but it gives you an idea of what to aim to save.

Note that you can also claim legitimate expenses on your income tax self-assessment even if you don't set up a limited company and just operate as a sole trader. There are advantages and disadvantages to both methods.

Also, dependent on what you are actually freelancing as, you may need to prove you're working for multiple clients. If you're just working on the side for one person all the time, the taxman might consider you're really employed, not freelance. I'm a self-employed Admin Assistant/PA and had to prove I had multiple clients before my accountant would agree to go ahead with self-employment registration.

Completely agree with you these sites are good for freelance and side income

Have you tried any of the other sites mentioned further up the page? I'm not sure how many are still going four years later, but if any are, that might be taken as a good sign.