My Wife and I started with a Sonos Beam in our Dining Room and later added surrounds (One SL’s). The two additional speakers do certainly make a huge difference if watching surround sound Movies like those available on Netflix and Amazon Prime etc; but if watching the ‘news’ and general stereo TV channels, then they make little difference.

I still love having the ‘surrounds’ though, even just for the occasional Movies, as it is a real cinematic experience. The surround speakers also make a difference when playing music audio in ‘Full’ music mode, as it widens the soundstage and fills the entire room with sound from all sides. 

We added the Sonos Sub last of all to our setup and that made a ‘surprising’ difference to all types of audio, plus there is a TV ‘Night Mode’ option to ensure our neighbours are not disturbed by the low frequency output in the evenings. 

That was all setup and done in our Dining Room a year or two ago, but in more recent times, we opted to get a new LG OLED TV and the Sonos Arc for our Lounge.  We had no hesitation adding both surrounds and a Sub in that instance… and no regrets for that room either.

Note, if you were to find the two additional surround speakers a bit of an overkill for your size of room, they can still easily & quickly be unplugged and utilised in other rooms of the Home instead …so there is always that option open to you as a bit of a ‘fallback’.

Try with just the Arc and Sub for a while, and get familiar with the sound. I’m sure Sonos still do the home trial scheme in the uk so in a few weeks, buy the Ones and add them to your system. (Try them as room speakers too.) Decide to keep them or return them. 

Many thanks both for your thoughts. 
Having raised again yesterday, I’m not going to be able to negotiate getting rears with my wife for now, so will be getting just the bar and sub initially - just a bit frustrating that relative to what we are spending, adding the rears is minimal cost - but the issue is more about room aesthetics. 
We have ordered a couple of Roam speakers to use around the house and in the garden, so just thought - at some point, could I do a trial and add these to the TV setup as L & R rears, to get an idea of what this would add to the surround sound ?
Ken: interesting what you say about the type of TV determines need for rears - we actually watch more more general TV and documentary type films (David Attenborough etc) and not many “blockbuster” type films on Netflix and Prime (also have Apple TV+ from a free trial, no idea why I don’t cancel now I’m paying, there’s so little content!). So maybe rears will have minimal effect. 
Maybe if we do watch something a bit “larger screen” I can see if I can add the roams as rears to the TV surround setup, to see the effect. 
Your other point about listening to music on full music mode is interesting - I can try that out, with the wife’s favourite track - The Last Resort from Hotel California!

@JustinD,

Just to let you know in advance that the portable ‘Roam’ speakers are not currently supported for bonding as surrounds to an Arc, or any Sonos HT setup, so sadly you won’t be able to test the full ‘surround’ option using those particular speakers. The Sonos ‘One SL’ or ‘One’ will likely be your best choice, if you do decide to explore the option later.

Back in the day, when HT had arrived, I incurred domestic displeasure and wired a full 5 speaker system in my living room - circa 1998. 

After some years of that, I realised that I could get most of the sound effects I needed for even movies like Apollo 13 with its lift off etc, from just one pair of quality stereo speakers flanking the TV using a stereo amp - because of how good those speakers were at bass delivery, even a Sub was not needed.

I now watch all my TV using sound from similar two speaker set ups, even in my dedicated to TV room.

Based on the preceding: in the case of dialogue heavy movies and documentaries, I doubt that even a Sub is needed, let alone the rear speakers. An easy way to check this out is by running just the bar when you get both in and seeing if the absence/presence of the Sub matters. And even if it does, note that the effect of rear surrounds for such movies will be lower than Sub effects, on the rare occasion these occur.

In my case, if I really want to experience the full monty for effects heavy blockbusters, I go to the nearby multiplex. In any case, the full experience from such cannot be had at home without annoying the neighbours.

Summary: I believe that you will not get much value from rears given what you watch, and for the occasional times you venture into blockbuster territory at home, the Sub will suffice.

PS: I agree and am in a similar boat with Apple TV+. Other than Ted Lasso, it sucks. It is the inertia that keeps me from cancelling...

Much appreciate that information - I can imagine the hours I might have spent wondering why it was no working!!

That reassuring to know!  It make me feel better that I am probably not missing out much on the speakers that have been vetoed by my wife!!

Many thanks all for your helpful comments

Justin