Definitely agree with Mike's last comment.
Don't confuse a bookkeeper with an accountant though. As Joe said the more of the preparation you can do the cheaper your bill will be.
An accountant is there to keep you on the straight and narrow and will advise what you can claim for and in what proportions (for example use of home for office). An accountant though is on your side and will help you claim for things you may not have thought of. Whilst you can do your returns yourself the tax office will only ensure it is correct, they will not go out of their way to show you ways of reducing your tax bill.
Your accountant will also be able to advise on your own personal tax return which will also need to be done as well as your accounts.
If you go limited you must by law have your accounts signed off by a chartered accountant but they will also be able to help you with other statutory returns.
A good accountant should be able to save their fees in reducing your tax bill by an even larger amount.
You can claim for anything you use for your business but not everything will be allowed. You can even claim for the cost of your Christmas party subject to a maximum of £150 per person (which may include a night away in a hotel) providing all employees are invited; which is precisely what Doctor Carpet and Nurse Carpet (ie my wife) have done in the past as we are the only employees!
Whilst one-man accountants can be cheaper if they work from home there are also down sides. I have known of accountants who have become unavailable, or been on holiday when you need them, disappear of the face of the earth, lose your books etc. etc. Obviously a large firm shouldn't do any of these and can often bring greater expertise to the mix. Often a larger firm can carry greater weight in any dispute with the tax authorities.
Beware also of accountants who offer a fixed deal as opposed to an estimate (like we will often do as a guide price) as their hearts (and wallets) will not be particularly interested in you if there are problems.
Claim for things like chemicals, stationary, advertising, repairs, postage. I have a separate business telephone line so there is no argument about what percentage of calls made on it are business or personal. The same goes for vehicles. Any capital expenditure eg machines can't be written off 100% against tax in the first year, they are written down over a number of years to reflect wear and tear.
As you can see the list goes on but I hope this answer gives you a flavour of the issues involved.
Roger