Timing matters more than most people realise, because relief lands in the period of purchase and a few days either side of your year end can move a deduction by a full year. Buy just before your year end and the relief comes off this year’s profit; buy just after and you wait twelve months for it. Annual investment allowance means most plant and machinery is relieved in full immediately rather than spread, so the sums involved can be significant. The obvious caution: this is a timing benefit, not free money, and buying something you do not need to save tax leaves you poorer by the net cost. Also note that cash basis accounting changes how this works, and that cars follow their own rules entirely, based on emissions rather than the general allowance.