Unit vs wall mount is swings and roundabouts. A unit solves your cabling issues. However, a lot of what's plugged in to the back of the set remains on show even if tidied up a bit. However, it's easy to change stuff in the future.
The chimney breast is likely to be the focal point in the room, but might be facing across the short axis which in smaller rooms can feel like the TV is on top of you. The chances are that TV sizes will only continue to increase. As they do though it becomes harder to hide the artefacts from upscaling standard def TV. Some might reply that they only watch the HD channels, and that would be a valid comment if it could be guaranteed that all the content on those HD channels was indeed HD in nature too. The thing is though, there are a lot of reruns, and those programmes weren't shot in HD, and there's only so much that the TV stations can do with scaling. During lockdown, there were a lot of old footy matches played out in place of the cancelled matches. Those were very far from HD quality.
Regarding TV height, this might be useful. Forf reference, 42" is the average eye height of a seated adult if they're not slouching.
This is the recommended positioning, and it works for screen up to 60". If the screen has to be a bit higher then a tilting wall bracket can be useful in helping to angle the TV picture towards the seating height.
Once you get above 60" then the recommendations start to change because the bottom of the TV image gets closer to floor level with a centralised set up. We use something like this instead:
Here, the eyeline is levelled at a point 1/3rd the height of the picture. You'd also want to use a tilting bracket. It doesn't need to be much to square up the picture, and most brackets allow up to 15 degrees which is plenty of adjustment range.
Something important to consider is the position of the seating relative to the screen, and more specifically, the type of LCD panel used, if indeed you're using LED LCD. Some brands (Sony, Samsung, Panasonic amongst them) use a type of panel called a VA (vertical alignment) which has the advantage of better black level than the alternative type (IPS), but at the expense of some colour shift/colour loss when viewed at 20 degrees or more off axis.
With a corner room layout, more of the seating is generally in a tighter viewing cone than when a TV is mounted and firing across the short axis of the room.
The alternative LED LCD screen technology is called IPS - In Plane Switching. It has wider viewing angles but the black never really gets black. There's always some light leakage through the panel. LG is the best known user of IPS panels, but because of the way the TV manufacturing market works, there are occasions where panels are sold to what appears to be rival brands, so you could end up with an IPS panel in a Sony or Samsung.
Check out your own TV's viewing angles. Make sure you're at eye level with the screen, then move off to view from progressively wider angles.
If you're still going with chimney mounting - and that might well be the best solution for you so make an informed choice based on what you and your family need rather than being influenced either way by us - then for cabling I would also think about the audio side of things. Flat screen TV speakers are generally pants. A sound bar can make a world of difference. The sound bar will need power and an audio signal of some type. The audio feed could be on a headphone jack for budget sound bars, or an optical for something in the £60-£200 range, or via HDMI ARC for sound bars at £100 upwards depending on brand and features. (Yes, I'm aware there's some overlap. That reflects the reality of the TV sound bar market. It's not cleanly divided.)
For routing the cables, definitely go with the piping idea. 68mm diameter guttering pipe is cheap, and there are 112.5 degree angle joints which will help make a smoother arc across the back of the chimney. Finish off with a brush plate covering the pipe exits at eaither end. Add a pull string to make running the next cable easier.
Go with power, enough HDMI cables to cover your needs, aerial and maybe Ethernet if wireless is a bit ropey in the house.
For the sound bar you'll need a power socket and audio connection as detailed above.
Good luck and come back with any further questions before you set to with the actual project.
