The official Royal Mail policy is this:
"We aim to deliver to addresses in urban areas by 3pm every
working day and addresses in rural areas by 4pm every working
day (exceptions are made for remote areas with limited transport
links such as the Highlands and Islands of Scotland).
Whilst our aim is to meet these times wherever possible, there
may be slight delays in some extreme circumstances, e.g. traffic
delays or adverse weather conditions."
https://www.royalmail.com/sites/default/files/docs/pdf/106831_col_del_pdf_02.pdf..unpublished
However, just like many other businesses, Royal Mail is likely to be suffering both from a shortage of lorry drivers to move mail between depots and also with staff self-isolating after testing positive for Covid. So deliveries somewhat later than their normal targets are probably to be expected.
In years gone by, I've received deliveries well into the evening during the peak Christmas period but I've actually been getting surprisingly early deliveries recently. For many years the postie has always delivered to my house within a few minutes either side of 12.45pm. However the local sorting office seem to have changed their delivery routes recently, resulting in me now getting deliveries around breakfast-time each day.