Apple Extreme extended using Ethernet switch

Note.......it is confusing and unintuitive to most users, but using Ethernet connections, the "remote" AirPort Extreme and AirPort Expresses would not be configured to "Extend a wireless network".

Post back for more info if needed.

As expected, AirPort Utility is correctly displaying the fact that all of the AirPorts are receiving their Internet connection from another device........(the Sky router in your particular case)

If........one of the AirPorts was acting as your "main' router providing the Internet connection, then the display in AirPort Utility would look something like the example below, except all the other AirPorts that are receiving the Internet information from the "main" AirPort would appear side by side next to the Time Capsule like the display of your network above.

Will this topology work?

Yes, assuming that you will not be using the WiFi signal from the Sky Router and understand the other compromises below.

Any issues to be aware of?

WiFi conflict between the Sky router and the "main" AirPort Extreme if you try to use the same WiFi signal on both devices. Best to turn off the WiFi signal on the Sky router.

Will this affect performance?

See "issues" above and "AirPort Express Ethernet Connections" below.

Any makes / models recommended for the Gigabit Ethernet Switch

Cisco and Netgear are known for reliability, but there are other good choices as well. TP-Link might be the best "bargain".

The Sky Broadband Router, Airport Extreme (main) and Gigabit Ethernet Switch are all in the same room. I have a number of devices in this room (TV, PS5 ...) that I want to connect using Ethernet cable. Would it be best to connect them to the Sky Broadband Router (which has 3 spare Ethernet ports), the Airport Extreme (main) or spare ports on the Gigabit Ethernet Switch?

Best to connect to the switch as first choice, the AirPort Extreme second and the Sky router third.

For the extended Airport Extreme and Airport Express - is it ok to also connect devices to them using Ethernet cables

Yes.....BUT......the Ethernet ports on the AirPort Express are only rated as 100 Mbps, not 1,000 Mbps or Gigabit, so any device or any WiFi device that connects to an Express will be limited to a maximum speed of 100 Mbps.

Is there a limit to how many Airport Express I can connect to the Gigabit Ethernet Switch?

No

why is it best to connect devices first to switch

Networking theory says this is "best", because it is the simplest combination and easiest setup to troubleshoot in the event of any issues. In most cases though, it does not really matter whether an Ethernet device connects to the Sky router, the switch, or to the AirPort Extreme.

By connecting the switch to the "main" AirPort Extreme, you are in effect adding additional Ethernet ports to the "main" AirPort Extreme.

Is it better to get a managed or unmanaged switch?

Unmanaged is by far the simplest way to set things up, unless you want to go through the process of programming a managed switch to set up authorizations for each device that connects by using the unique MAC Address of each device. I would only consider a managed switch if I were concerned about the possibility of unauthorized persons connecting to the network using Ethernet.

Regarding the earlier question about a limit to the number of AirPort Expresses can connect.......there is a limit.....but it is in the range of 250 Expresses, so for practical home applications there is not really a limit.

Only Ethernet devices connected to your AirPort Express base stations would be limited to 100 Mbps connections. Devices, connected to the router, Ethernet switch, or Extreme base stations would get 1 Gbps connections.

Going back to managed switch. One of the purposes of utilizing one is to create Virtual LANs (VLAN). The advantage of doing so is to segregate traffic on your network. However, for this to work, but the switch(es) & router must support VLANs. So, in this case, if you wanted to add IoT devices on your network, currently with your set up, those devices can access your "main" (and only) network. As you know IoT devices can be insecure. Meaning that they can get communications from the Internet that you did not intend for them to get ... like malware. With a VLANs, you can place all of your IoT devices on a single VLAN to prevent them from accessing your main network.

the Ethernet ports on the express are limited to 100mb. Does that also apply to the Wi-Fi?

Yes, since the WiFi signal on the Express is derived from the Ethernet signal......which is limited to 100 Mbps

Good luck!