@merlin777
To achieve what you ask, you will need to configure Deco in Access Point mode. By default, when you configure first Deco which will become Main Deco in your Deco mesh, it'll run in Router mode.
After you've successfully completed Main Deco configuration, in Deco app under More/Advanced make that change:
Then, add remaining Deco units to your Deco mesh according to the directions in user manual that came with Deco set.
In that configuration, you can use second (free) Main Deco Ethernet port and both Satellite Deco Ethernet ports as connections to your home network. It'll work seamlessly. For example, if you have Deco unit near PC, XBOX, SmartTV - you could plug them to free Ethernet port of that Deco unit and they can join your home network as if they were connected to Virgin hub3.
You can also continue to use the Ethernet sockets on the virgin hub.
The choice of M4 or S4 is appropriate, and you can also consider M5. They all claim support of about 100 wireless devices.
Deco M5, you can wall mount it in narrow places or even ceiling mount it, using wall mount holder you could get separately from the third party. I got mine from Amazon.
Be careful when it comes to WiFi coverage promoted. That applies to any WiFi mesh. Some advertise coverage of many thousands sq.ft. for 2-unit set. More realistic, if you are looking for good WiFi coverage, is one WiFi node per about 500 sq.ft. of house space.
You shouldn't be buying that many Deco units at once, but be prepared to add more if you expect 3-unit Deco M4 to cover a house 5,500 sq. ft. large.
I strongly recommend buying WiFi mesh, not just Deco, from the place with good return policy. You might need just a week or two after WiFi mesh deployment to figure if it works for you or not. Some people, like me, end really happy with their Deco WiFi, and some face serious issues they can't overcome even with help from TP-Link Support. One month return policy is a must.