W H Smith have been struggling for years. They'll almost certainly go the way of Woolies within the next few months.
Post Offices vary widely. Some principal offices have never really done anything other than providing their core services. (i.e. they've offered very little, if anything at all, on the 'shop' side of things). Some sub-POs operate primarily as shops, where the postal business is just an extra service. They each allocate their staff according to the demands on each side of the business.
Queue lengths seem to vary greatly between post offices. Last Monday I was working on a temporary job in Southend. I wanted to post my grandson's birthday present but when I went to the main P.O. the queue was out of door and down the street. I asked someone who was leaving how long they'd queued for and they said "just under an hour". That was no use to me, since I only had half an hour's break.
Two days later I tried again to post that present. I walked into the main PO in Halstead (Essex) to find that there were three staff all waiting for customers. (i.e. no queues at all). Interestingly, that Post Office is within a very quiet branch of W H Smith!
Chris