IR35 is a tax law that basically describes certain contractors as "disguised employees"

If you get hit by it then you are going to have to pay a LOT of tax, including both lots of national insurance (employers and employees) but if you are outside it then you can pay yourself a lot of the money in dividends and completely aboid NI on the bulk of the money.
Also if you are outside it there is no cap on what you can expense, anything that is a potential business expense is valid, but if you are caught by it35 they put a cap of 5% on it, you cant even claim above that for stuff like training courses.

If you are looking for a good accountant and are outside of it, i can really strongly recomend the guys and gals over at http://www.lowtax.co.uk