Nana, the best place for this kind of question is the Society of Authors: they have excellent leaflets which you can buy for a few pounds, and they'll advise members on the phone.
Society of Authors Publications
If the Inland Revenue accepts you as a freelance writer then yes, you can claim expenses like subscriptions - and many others, including business mileage, and a proportion of the cost of running your house - against your income. As far as I know you just have to tell the Revenue that you are doing freelance work. It was a good three years before my accountant said that I'd need to show some freelance earnings if I wanted the Revenue to go on accepting me as one. If your expenses are greater than your income from writing, then you can carry that loss over to the next year, and so on. Comes in handy for reducing the tax bill once you do start making a little profit.
Just for the record, though, technically prize money is taxable too, if you 'solicited' the prize; i.e. you entered the work yourself, rather than it being awarded from on high, or your publishers entering it for you.
Emma
<Added>Re: the laptop, the rules about office hardware are slightly more complicated - I think you're only allowed to set a proportion of the price against tax in the first year, and more in the second. But it's still worth doing. Library fines, accountant's fees, coffee/food/hotels while you're reasearching away from home... It all adds up.
Authors tax affairs are an odd corner of the tax laws, though: many general books/accountants/helpful brothers-in-law don't know what they're talking about.
<Added>Oh, and once you're in business, you can offset training costs too. Could be a way of easing the financial pain of paying for a Masters...