Jimbocz is right to recommend an accountant. But there are steps you should be taking yourself.
First, go here and read up:
https://www.gov.uk/log-in-file-self-assessment-tax-return/if-youre-already-registered Click on 'register if you're self-employed and follow the instructions under 'if you haven't sent a return before'. It is not advantageous at £14K to register and file as a limited company, so you will be a sole trader.
If this is for the 2016-17 tax year, you've got time to sort yourself out. Your online return for 1 April 2016-31 March 2017 (which is essentially how things are calculated for accounting purposes) is due 31 January 2018. You also must pay self-assessment tax and Class 2 and 4 NICs by this date. You might also have to pay a 'payment on account' towards the 2017-18 tax year by this date, depending on if you are continuing to work as a sole trader in the new tax year. (You may, of course, pay before then.)
Once you are registered on the HMRC site, you will be able to walk through the form, get the calculation, submit the self-assessment return, etc. I encourage you to do this before the 15 June US expat tax-filing deadline, so that you have proof to send to the IRS and can comply by getting the foreign earned income exclusion (FEIE). It will also help you decide if you want to pay for an accountant.
I can tell you how I do things as a sole trader, to give you a point of reference but you by no means have to follow my model. I know other sole-traders who operate completely differently.
1. I set aside a percentage of each payment I receive from a client, socking that money into an ISA so I don't have to pay tax on the money I'll use to pay tax.
2. I keep all my business receipts -- e.g. Adobe Creative Suite subscription, Microsoft Office subscription -- in a digital folder. Admittedly, I don't have very complicated expenses, so it's easy to keep track.
3. Come mid-April, I create a Google Drive for my accountant. Inside, she gets my income spreadsheet, so she can see exactly how much I've brought in over the course of the tax year. She also gets my various expense receipts in an organised manner.
4. By the end of April, she comes back with how much I'll owe, and the payments on my March invoices come in. She submits to HMRC on my behalf (there is an online mechanism through the HMRC site for approving someone to be able to do so). I pay her (£180).
5. When HMRC recognises that my return has been filed (mid-May to early-June-ish), I pay the tax due by 31 Jan. This is because I do not work in arrears, and have the money set aside already. I know other people who work in the new tax year to pay the bill due in January, and it seems to work for them mentally, so long as they pay by HMRC's deadline. This is not an approach that works with my psychology.
6. In early June, I fill in the IRS FEIE form and 1040 (this year will be for 2016 US tax year), ultimately owing nothing to the IRS. Post the forms before the 15 June deadline, et voila! Another expat freelancer tax year done and dusted.