So, books are zero rated, a writer is given a share of each book sold, but if his share goes over the VAT limit, he has to start paying it on his earnings. Is that correct?
Er... no. You don't pay VAT on earnings. You pay income tax and National Insurance on earnings (or corporation tax if you do turn yourself into a limited company - but I think HMR&C will be looking closely at that kind of thing in future**). VAT is
charged by the supplier/producer when he sells, and therefore
paid by the consumer when he buys.
I'm happy to be corrected, but I really don't think VAT enters into the equation for a fiction writer, no more than for any other person buying ordinary stuff. That is, you will pay VAT on your pencils and lawyer's fees, but you won't be charging it to anyone. In any event, stop worrying about it now. When you land your big deal, you can pick your agent's brains. And if you get a big enough deal you ought to be speaking to your accountant anyway about other tax-evasion dodges... er... I mean... legitimate ways of avoiding tax in all its multi-various forms.
TEIN, you're right, it isn't the invoicing as such which creates the VATable bit, but the supply of the good/services - but a VAT registered business must provide a VAT invoice. So far as payment is concerned, as I understand it, your agent chases the publisher and receives the whole amount due, whether by way of the advance or any royalties; the agent then deducts his/her 15% or whatever and accounts to you for the balance together (I assume) with an invoice - since the agent's fees are a deductible expense when it comes to filling out your tax form (because you have to disclose the gross earnings, not the nett you actually receive into your hot, sticky hands). And if the agent earns enough, then presumably his/her invoice
will carry VAT, since he/she is providing a service.
** and it's of note that Terry Pratchett hasn't turned himself into a limited company (though he and his wife have a formal parnership with regard to copyright at least).