Historically, over about the past decade up until the pound took a bath following the Brexit vote, Apple have charged very close to the same amount for their products in the UK as in the USA, once one takes VAT into account. The difference for most products has usually been within a few percentage points. Admittedly for some products the difference has been larger, but for their more expensive items the prices have tended to be close.
Occasionally currency fluctuations have meant that some Apple products have been cheaper in the UK than the USA (once VAT is considered) - that was the case when I bought my MacBook Pro a few years ago.
Apple tends to only very rarely adjust their pricing and will generally ride out currency fluctuations, preferring instead to maintain consistent pricing for their products. The adjustments they have made to GBP pricing were done many months after the pound had devalued. Typically their adjustments are done on much longer timescales (years) but as the pound dropped off a cliff it's not surprising they've made these adjustments faster.
In this particular case, this is an automatic re-pricing of apps so that developers around the world won't be taking a bath on product sales to the UK. Those developers can choose to re-price their product at a newly introduced lower price band to let said products continue to be priced at 79p in the UK should they choose to do so.