I've finally decided that 3G only will be the future of my phone unless I need 4G speeds, if someone over at XDA can't make a 3G/4G mode.
Tried testing the phone (S4) again. This time, the phone dropped to EDGE twice - once was near the railway bridge at Irchester (Northamptonshire), and the other time was on the main road into Rushden/Higham Ferrers.
The first time was completely unnecessary - right next to the railway bridge is an Orange 2G/3G mast, that is still broadcasting 3G under the Orange code. Why on earth did it ignore the 3G, and go to 2G? The 2G was EDGE, so probably from the recently upgraded water tower mast in Rushden, which I assume is where the 4G for the area is coming from.
Second time, again jumped to EDGE from 4G. Why it did this, I do not know, because there was clearly an MBNL 3G signal from the Higham Ferrers mast! By that time I'd had enough and forced the phone to WCDMA only... surprise surprise, 28% (8ASU) 3G signal.
So why on earth is this phone preferring 2G over strong 3G signals? It was bad enough when the phone dropped from 1-2 bars 3G (or ignored said signal), but ignoring strong 3G?
I'm asking XDA Developers if they can get a 3G/4G mode for me. And if I can get said 3G/4G mode, then that will be used permanently. And the day that the UK switches off 3G (which will probably be after my phone has given way) will be the day that the S4 will not be able to take a voice call.