In reply to Joss:
Hi Joss, awesome that you have your first commission.
There are some really complicated numbers and terms being thrown about here but unless I am missing something then it seems simple to me.
I have worked self employed for myself for 8 years with a small sideline including photography so this is where I am coming from.
You have a job. You pay taxes on your income through that job. (through PAYE probably)
You also have a 'piece of work' that is going to get you money. They are earnings and need to be taxed.
You found that work yourself, therefore for that time you are self employed. That is what you need to register as. Self employed sole trader. {FYI VAT has nothing to do with it and you dont need to set yourself up as buisness, it can just be in your name}
You need to keep and account (excel sheet perhaps) of how much you earned and when (IE the invoice you will send)
then you can do one of two things come the end of the tax year:
Fill in the online self assessment: it has all the bits you need to know about declaring income from other jobs etc. (for you this should be simple if it is only one job or series of jobs for this client)
or
you give your account sheet to an accountant and he files it and charges you a sum. (I use this method but when your main income is sole trader self employed from lots of sources and you claim things back agains tax then it becomes a bit more complicated)
I would suggest thats the way to go.
Hope that helps
Rob
(btw - I would thoroughly recommend that you look up the website shakodo.com for photography pricing to make sure that you are not selling yourself short like a lot of amateurs entering the semi pro world do- they are really helpful and a wealth of advice)