It's not clear whether your post relates to a claim against an employer, or against the tax man.
If you're hoping to get money out of your employer, you'll have to check your contract. (Some companies insist that their staff pay for essential items of clothing. e.g. easyJet staff have to pay for their own uniforms. That can be perfectly lawful if it forms part of the contract of employment).
If you're referring to a tax claim, you need to be aware that you won't get the full cost of any essential items refunded. It's simply that you're not taxed on money used to buy those items. So, if you have to buy �100 worth of items which are essential for (and solely for) work use, you'll pay �20 less tax (assuming that you pay tax at the standard rate).
If you're self-employed, see the information here:
http://www.hmrc.gov.uk/incometax/relief-self-e mp.htm
If you're an employee, see here:
http://www.hmrc.gov.uk/incometax/how-to-get.ht m
Chris