And you don't get to offset all the cost of the things you buy for use in your business - you get Writing Down Allowances. So if you buy a toolkit for use in the business for £100, in the first year you can claim WDA of 25% of this (£25) to set against your profits. If you don't make a profit it is of absolutely no use to you (except that losses can carry forward until you do make a profit a few years down the line at which time they can offset that profit and reduce the tax payable then).

Some assets get special rules from time to time - eg 100% first year allowance was given on IT equipment a few years ago to help encourage high-tech investment.