Yes, but only 30% of what he pays for it will be deducted from his tax bill, not the total cost, which would in fact mean everything would be free and we all know that isn't the case don't we children 
How come only 30 percent ? What about van finance etc I don't understand why it would only be 30 percent fir something you actually need that's of no personal use at all say you earn £16.000 a year and you have 2k worth of recipts fir fuel poles insurance whatever else surely you will only then get taxed on the remaining £14.000?
If you take your example, and assume tax is not due on the first £10,000 and that you're paying tax on the top £4,000 of that profit at 30%, so
£1,200 due in tax.
Then you decide to spend £500 on a new pole. Turnover stays at £16,000 and expenses are now £2,500. Now profit is £13,500. You're taxed at 30% on the profit in excess of £10,000, so £3,500. Thus, the tax you pay is now
£1,050.
Your tax bill has declined by £150, which is 30% of the cost of the pole.
It's a common misconception that something offset against tax is free (particularly by people not running their own business who think we're on some kind of gravy train) but it's not true. What you effectively get back is the tax on the purchase.
Hope that clarifies.
Vin