RJ45 ethernet socket question

Yes. Only things worth noting are if you are taking the cable outside the house you might as well use outdoor cable and I'd also think about using shielded outdoor cable. Ideally put cable in some trunking.

Consider get second run up to soffit, then into attic - will give you option to distribute network from above, e.g WiFi AP(s) can be there to cover entire upper floor.

Yes, double cable and double plate. Have to be individual connections to maintain 1000Mbps link.

Depend on house layout and your goals , you could have 1 run to attic(or other central location) , install network switch there and distribute network from there.

How fast is your broadband? That switch if only 10/100Mbps which means if you have an internet connection faster than 100Mbps you won't be able to use all your internet bandwidth.

Minor nitpick but in your proposed layout:

Broadband Router---RJ45-----RJ45 socket------------RJ45 socket----RJ45----TV

RJ45 should probably be Cat 6. RJ45 is the connector, cat 6 is the cable

As regards a switch, you might not need one at all - your router should have 4+ rj45 connections on the back.

That will allow you to feed 4 other devices e.g. sitting room tv, upstairs playstation tv, upstairs wifi AP (if desired) etc

I'm wondering because there is that many cables behind the wall plate cover could they be looped from one wallplate to another rather than each cable going back to a central location, considering the build goes back to c.2004. That was the recommended reci wiring scheme back then?

If run as a radial and you have power at the points you could still potentially use them but would need a switch at each point to take the connection from the previous one, distribute to that room and send it on to the next point. You could end up with a daisy chain many levels deep though if there were a lot of runs.