Capital allowances, from what I recall, only applies to the tax burden, and not the tax rate. It could be possible that it is a form of subsidised tax rate specifically for Royal Mail. But from a further look at the statements, page 57 shows the adjustments, with 19% being the effective rate. So possibly an error on OPs part.

edit: nevermind, effective tax rate is 7.6%, down by 11.4%, due to remeasurements of deferred tax, net pension credit interest, and uncertain tax provisions.