This has to be one of the most misused yet repeated statements on this site and in lots of conversations down the pub.
If you spend money on something and it can be offset against tax, remember that you're still paying (if you're a basic rate payer) 77% of the cost of that item.
For proof, imagine you net out £10K of profits above your allowance. You pay £2,300 tax (at 23%) - net salary after tax on this portion of your income = £7,700
Now, if you spend £1K on something or other, then your net profit drops to £9K and you pay £2,070 in tax - net salary on this is now £6,930
So £7,700 - £6,930 = £770, so you have paid 77% of the cost of the item out of your pocket.
So, next time someone tells you that you're OK, you get "X" free because you can offset it against tax, shoot them with my approval.
Vin