The current 'collection windows' are here:
http://www.royalmail.com/personal/help-and-support/when-do-we-collect-from-postboxes
However if, say, a van will completing the last collection on its round at 1730 then (obviously) the postboxes before that one will have earlier times for their final collections. (Typically around 1600 to 1700 around here).
Postboxes within private premises (such as lage supermarkets and the Wythenshaw Forum Centre) are subject to separate commercial arrangements between those premises and Royal Mail.
The system is simply responsive to customers' needs. Commercial organisations need collections up until the end of their working day, so Royal Mail collect directly from their offices up until 1930. However very few people now send urgent letters (prefering to use email instead), so there's no point in sending out vans to empty postboxes. Further, most people now have transport to get them to a sorting office on the rare occasions that they need to post an urgent letter in the evening.
The Post Office (which, of course, I recognise is now separate from Royal Mail) has been remarkably good at modernising its services in recent years, with loads of post offices around here open 7 days per week (often fom 0600 to 2300), so it's not all bad!