If you are in full time employment and your employer allows you to do this, I'm not sure there is anything you can claim off HMRC.
Look at it this way - you're not having to pay travel costs to/from your normal place of work.
I think you can work out what extra it costs you to run your house when working at home rather than being out. It's up to you to decide what that is.
If (big if) the taxman accepts your calculations then he can adjust your tax code to reduce your tax.
There is a snag in doing this though - your house then loses a proportion of the principal private residence capital gains tax relief when you come to sell it.
Depending on which accountant you listen to could be different advice - my accountant says (but bear in mind I work from home permanently):
Add up all your bills (mortgage, water, gas etc) and divide it by the number of rooms you have in the house (including bathrooms & kitchens etc). This is the amount I can claim back on my self assessment.