It shouldn't be a straight 2.5% rise anyway, as it's only the VAT proportion of the price which will rise anyway, not the whole price.
So that £99.99 CPU including 15% VAT should be £102.17 including 17.5% VAT, not £102.49 as you suggest.
The actual rise in price inclusive of VAT should be just over ~2.17%
Going back to the general topic of jumping in to beat the rise, it's only really worthwhile if there's an expensive item you've been thinking of getting (say a large TV, full computer system, sofa, Car etc). As a general rule with the price of IT hardware I'd expect that 2.17% increase to be quickly swallowed up by the natural depreciation we typically see in the industry (volatile RAM markets aside).