To start with the choice of the Arc… Have you ever considered to look at the beam gen2? Arc has more power and therefore is more suitable for larger rooms. When you like to think about your neighbours and like to listen at lower volumes, I believe the beam gen2 will be more than enough. 
You also need to consider the size of your TV. The Arc is 45 inch wide, so it would be nice it’s not wider than your TV. Also your TV needs to have an eARC-HDMI port to get Dolby Atmos.

Maybe you want to watch this one first, which can maybe help you decide:
 

About choice of Sub… When you go for the sub-mini (which probably will have enough power to fill the room with sufficient bass), you need to consider that when you want to go for a dual sub setup in the future, this is not possible, You can only add 2 subs, using at least 1 sub gen3 and another gen 3, 2 or 1. Dual sub setups helps you to equal out the bass, so there are less peak- and dead- spots. Believe it or not, your neighbour will have less notice when using this type of setup because it flattens out the peak area’s.
If you don't care about dual sub setup in the future, just go for the sub-mini, excellent choice for the size of your room I think.

This video explains you the benefits for dual subs:
 

If you like best sound possible, I would definitely go for surrounds. If you choose the one’s, there is no point in investing in One’s with a microphone, since those microphones will be disabled when you set them up as surrounds. The HT (Home theatre) system will only use the microphone in the Arc or Beam. Instead you are better off with 2 one-SL’s which don’t have a microphone.

You can buy original Sonos stands for the one’s/oneSL’s, which you can place at either side of the couch. Room correction (Trueplay) is very helpful to create a base line. It can only be done with an iOS device. After enabling Trueplay you can adjust the volume of the surrounds further down, to avoid the speakers yelling into your ears. I have mine wall mounted in about 3 feet from the listeners position also, and I found, reducing the volume by a -3 works great.

More to watch about rear speaker placement:
 

Another video for the complete setup (Beam gen2, Sub-mini, and surrounds):
 

Hope I did give you a bit of an insight and thoughts to consider for now. 

  Hey @JeroenVR, thanks for your reply! I appreciate your insight.

To start with the choice of the Arc… Have you ever considered to look at the beam gen2? Arc has more power and therefore is more suitable for larger rooms. When you like to think about your neighbours and like to listen at lower volumes, I believe the beam gen2 will be more than enough. 

I have considered Beam v2, I know it’s enough for my room size-wise, and the power output is enough, but isn’t Arc simply a better device? Would you still go for the Beam, if you weren’t worried about the price difference?

I’ve just watched the Peter Pee’s video that you linked to (I haven’t seen this particular one before). On one hand, his assesment that Arc gives 10%-ish better performance is very reassuring. On the other - Arc’s frequency curve is much flatter across all frequencies (esp. when paired with a sub), while Beam seems to be more bass-heavy while losing in the mid range, and weirdly doesn’t seem to benefit as much from pairing with a sub. Not a huge fan of that, but still - thanks for the suggestion and I actually might consider the Beam.

Also, no, sadly I have a 2018 LG OLED which comes with a regular ARC rather than eARC. My understanding is that I will still get Atmos, but with a lossy DD+ compression instead of lossless TrueHD.

If you like best sound possible, I would definitely go for surrounds.

Of course I would love to! And this is my main problem - I have no room for a left-hand side speaker. Could you please look at the pictures here, I’ve marked my possible locations for surrounds with red marks → https://imgur.com/a/F6rocUc

As you can see, my left-hand size speaker would have to be located on the sofa armrest, in the corner, about a foot or less from my hear. I know I can adjust the sound a bit, but does it actually make sense to put a surround speaker this close, literally on the sofa? 

Basically - is it better to have a surround with one speaker placed correctly, and another right next to my head, or rather not do surround at all?

Well… If money is no issue and you just want to get to the best possible, yes, then Arc is the best Sonos soundbar out there. It has the up firing Dolby Atmos speakers, which are bouncing the sounds of the ceiling. The Beam gen2 Atmos speakers are not up firing, which is a difference. Also Arc has 11 speakers, and Beam has 5. So yes, there is a difference of course. 

About the surrounds… I have looked at your suggestion about placement. I think you will not get the full potential when placing them like that. The ideal setup is playing them symmetrical at ear height. When putting left rear on the armrest and the other one at the end of the doorway, the left right will be dominating the sound. I don't think Trueplay is able to balance that out. Plus, the left rear is way below ear height.

That's why I thought about the speaker stands. They are designed, so you can slide the foot under the couch. That way you can create a symmetric setup and at ear height.
Another option is to wall mount them, using the flexible Sonos wall mounts. I see there is a bit of space above the curtain on the wall, where you can fit them. That way you need to mount both speakers at ceiling height, pointing them both to the listeners position. Although they are not ment to be mounted that heigh, the sound needs to come from behind, not above. Trueplay can correct that a bit, but not completely.
Third option, is to start with the Arc/Beam and sub and just try it out yourself. You can always add them later. But be warned… when you start with Sonos, your whole home will be filled with speakers before you know it .

About the sub.. If you go for the Arc, maybe it’s worth considering going for the Sub gen3 instead of the sub-mini. The Sub gen3 alone is more powerful that 2 sub-mini’s and you will have the option to expand the system to dual subs in the future. This week, the Sub gen3 is $150,- cheaper, which makes it like a no brainer with $599,- instead of  $749,-. Arc is $180,- cheaper this week also. They are practically giving it away 🥳 This Black Friday week started on 18th of November and ends on the 28th.

Edit:
I see you live in the UK. Have a look on this website for this weeks discounts.