Quite possibly, from what I have done so far with them, they are just allowances to reduce the tax burden relating to capital assets. But yes, I think they are incentivised via the government to encourage larger corporations.

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.

Possibly capital allowances, but that would affect overall tax payable, not necessarily the rate of tax?

Can someone explain the 7.6% tax rate?