My last post was posted as an "info request" but no info was needed over and above what you had already tole me.
As you started the freelance work in June 2014, your first tax return will be for the year ended 5 April 2015. I suggest you run your accounts to 5 April 2015 to keep things simple and avoid the double taxing of some of your first years's profit twice. Clearly, the Sub-contract fees will be a deductible expense. You can register as self-employed here
. Any tax and national insurance contributions for 2014/15 will be payable to HMRc on 31 January 2016. You may also have to pay instalments on account of the following tax year's liability but that will depend on the level of your earnings from your regular job.
As a self-employed individual, you will pay tax at 20%/40%/45% and Class 4 NIC at 9% (over a certain profit level) on your profit as you make it. If you set up a company, then you will only pay personal tax when you draw income out of the company in the form of salary and/or dividends. Dividends are an efficient way to take income from a company as they are not subject to NIC whereas salary above a certain level is. The company will pay corporation tax at 20% on its profits. Dividends are a not a deductible expense for a company whereas salary is.
Most people who work for themselves start out as self-employed to keep costs down and to see it things work out. Running a company comes with more onerous responsibilities and complicance costs so you may prefer to wait until you are generating a more signicant income before you start up a company but it is entirely up to you. Whilst you are employed elsewhere, you would not need to draw income from company funds thereby keeping your personla tax liabilities down which you won't be able to do if you are self-employed and still working as an employee elsewhere.
You can set up a company online yourself or use an accountant or company formation service to do it for you. Search on "company formation" for more information.
I hope this helps but let me know if you have any further questions.