Seeing as your two customers are both in business, i assume they are registered for VAT. If so, then you should register for VAT also so that you can reclaim VAT on your expenses, such as pc, print cartridges etc.
In any case, you need to register for income tax via the Form TR1 [broken link removed] You can also register for VAT with this form.
Any related expenditure such as computer costs, and portions of your heating and phone bills can be deducted from you income figure at the end of the year when you come to calculate your taxable income. You will have to try and estimate the % of your phone and light & heat bills which are business related and use that % going forward.