Monitor Input selection

I am planning to buy an iMac soon.
I would need to connect a Freesat TV box to the monitor . The Freesat box has an HDMI output. Would I use a Thunderbolt adapter cable to connect?
Also how do I switch inputs? ie is there a pop-up on screen menu?

The only way you can feed anything into an iMac is to set to Target Display Mode. This mode is primarily used with another Mac.

The iMac isn't meant to be used the way you want to use it..

input selection

Any freestanding monitor could be used to select 1 of 2 inputs.
Do you mean that with an iMac this is just not possible???
I would have thought that for £1000+ it would be a pretty basic facility?

Umm, the iMac is not a freestanding monitor. It's an all-in-one computer that just happens to have a really nice screen in front and all of the computer innards sitting behind it.

So rather than having a separate computer case and a monitor and all that cables that go in between these two components, you have a single piece of hardware that combines them all.

Show me another all-in-one computer that allows for what you want and we can talk about the iMac's limitations..

I had not realised the limitations of "all-in-ones". It's a pity the "Thunderbolt" connection is not more flexible.
I'll have to use the TV box separately.

Not really a limitation of an all-in-one, think of how you would/could accomplish the same thing with a standard desktop's connectors while not touching the monitor. You wouldn't..

Thunderbolt is just a conduit of communication, the restriction isn't there and has nothing to do with its flexibility. Thunderbolt can be used to connect to monitor (like the Apple Cinema) that have a TB port. The TB port can be used with adapters for TB->Ethernet, TB->whatever else and so on..

That actually speaks to a great amount of flexibility with TB not how rigid it might be.

You might want to better understand the technology that you are looking into before forming opinions about their abilities or inabilities..

Found this on the MAC website...
Can I use the iMac in TDM as a display for my gaming console, DVD player, or other device?
The Mini DisplayPort port on a supported iMac can receive DisplayPort compliant video and audio signals. Converters not made by Apple may provide options to convert other electrical, video, and audio protocols to Mini DisplayPort or Thunderbolt compliant signals. ..........

This seems to be what I need. I found one converter here...
Atlona (Dual Link) DVI to Mini DisplayPort Converter for Apple 27-inch LED and 27-inch iMac
but the manufacturer states.."My brand new Thunderbolt monitor doesn’t work with DP400. The connections look the same.
Unfortunately none of our products support Thunderbolt based Displays."
So I am now looking for a converter that would work. Does anyone have any info? (it would need to work with a 21" NEW iMAC (Yosemite).

update...am I right in thinking that the mini-dispay port on a new iMAC is an OUTPUT only and therefore the above comments on the MAC website must refer to older/different products? One manufacturer has stated that a Thunderbold converter has not yet been made so NO GO?

If you want to watch TV on your iMac, you are going to have to do it via the internet, or with a USB TV tuner like this:
https://www.elgato.com/en/eyetv/eyetv-diversity

The advantage of the TV tuner is that the software allows you to record TV programs. The above has dual tuners, so that you can record two channels at the same time, or record one and watch another.

I think the best solution would be to get a mac pro or a mac mini and connecting it to an external monitor that was designed to take multiple source inputs. An all-in-one like the iMac is designed as a dedicated computing solution not a versatile component solution.