From memory whether you are on line or not if you turn over/earn less than 15k you do not have to provide a profit and loss account or breakdown. Say for instance you are a childminder and you earn 12k, then instead of claiming this and that (wear of carpet etc, cost of toys) IR say you don't need to bother.
Now say You're a window cleaner, you earn 16k, then you do need a profit and loss account. ie total sales 23k cost of goods or services 5k, motoring diesel 1k, insurance etc1k equals 16k profit.
After the profit there are things like use of home as office and depreciation on vehicles.
Be warned though, don't take advice of idiots ( for want of a better word) like me on tax issues. The above is just the giste and may be incorrect.