I've been self employed for over 12 years, it's true you can offset many expenses against income.
If you're only doing it short term, not sure how helpful my info is, but here goes:
The 'bonus' of offsetting has its downsides, one being that with a low net income, it's hard to borrow money. My first mortgage offer was only 36k! The other downsides are having no pension, no additional national insurance payments, no holiday pay, no sick pay/health cover etc etc.
There are some upsides though, like more freedom, and you have to have the mindset of taking holiday as necessary 8-)
I don't claim any VAT as I'm not registered, and I think you have to be earning more than 57k to do that.. there are a whole can of worms in that area too. :-?
My advice would also be get an accountant and put aside a good percentage, like 25%, of earnings for tax and NI (which you only pay twice a year). This should be more than enough and a small nest egg will grow from it. It should cover fees and will grow more if you get refunds. Because you pay tax based on the previous years net earnings, this can happen quite often. I overpaid over a couple of years as my expenses grew but not my income, and I got a £4k refund

Also, if you can, put some savings away for income protection and long term. This way, you've got dosh saved for unforeseen and unfortunate circumstances.
Keep records of everything you spend, and on what, and the accountant will help you work out which category they count toward! On vehicle expenses I add all of servicing, tax, insurance and RAC costs - as I need my car for work. Then claim a percentage of fuel expenses as not all miles are for work.
I haven't got to grips with the recent trend toward online self assessment, but I gather it's easier, less time consuming and hence needs less accountants time (and cost!)
hope that's vaguely useful....
PS the cost of an accountant depends on how much paperwork you want to do yourself. I like to do a lot so don't pay him much. But as such he fills the forms in, does the chatting to the tax man, and knows which figures relating to assets, capital gains, ......to use!

He's good at kicking my arse to get returns in on time too.
