Here are a couple useful youtube videos on what you can claim - both specfically aimed at the self employed/ home workers.
The first from Crunch (I think an accountancy acompany) lasts around 10 minutes and gives a good overview of what you can and cant claim and also the pitfalls to avoid (i.e. never claim 100% business usage of a room). It also gives advice as to how to make your business tax efficient - a bit dry but worth the investment of time to get the overview.
The second is a webinar by HMRC on business expenses - this one lasts 40 minutes and needs a bit more focus ... but is the official version 'from the horse's mouth'.