Hi Cloud,
Spencergate's point re. the conditions of the rental contract are quite correct; I personally have never seen a tenancy agreement which didn't prohibit business use.
However, in reality, that would mean that nobody who rents their home would be able to work from home, and that really isn't the intention of the clause in the contract which, as it is with my tenants, is really there to stop anyone from turning the house into a business premises with customers coming and signs being put up... etc. I've never heard of anyone being evicted for breaking the terms of their tenancy by working from home.
In terms of the expense, opinion is divided on this one and you will have some advice that it cannot be done. However, the other side of the argument (which is the side that I tend toward) is that it is appropriate to set against the income of your business any expenses which have been incurred in generating those revenues.
So in your case, if you set aside one room out of four purely for the use of your business, then it is not unreasonable to allocate 1/4 of your rent as an expense in the business. Of course if you also use that room for non-business purposes then you'd need to reduce that proportion.
In addition, it is common to include an allocation not only of the rent, but also of the heating, water and council tax costs.
The key issue is to be reasonable here, it's not about 'getting back as much as possible', it's about making a fair claim for the reasonable costs which you have incurred in the running of your business.
If you're moving out of commercial premises then, assuming your turnover was constant (which I suspect is unlikely, but that is a separate matter), your profit will be higher and HMRC will be receiving a larger remittance anyway.