Hi James,
Firstly; may I wish you a warm welcome to the CiC forums from me.
I would be wary of free-form Googling as a learning path for you - sure; there's a lot of good stuff on the internet, but also some 'less good' and finally some is just downright wrong. If you don't have much of an understanding, I fear you risk wasting time learning from what I'll call 'well presented fluff' (or even rubbish), rather than fact, because you won't be able to separate the wheat from the chaff. I recommend you find somewhere reliable, for example the tutorials on this site and work through them - there's much to be said for receiving a consistent approach. Random internet readings and viewings may confuse, due to the different presenter's differing styles of writing and presentation and emphasis on what they think is important. If you find a presenter of YouTube content that you 'get on with', stick with them, for consistency, there are a few I could recommend.
If you teach Skiing (from your web site), you probably know the point I'm labouring to death here
If these were shot before sun up, in mist, I'm not surprised there wasn't much light, resulting in a high ISO being needed - and that still not being enough to expose properly given the effect of mist/fog on metering.
I assume the head shot is just showing us a 100% view to demonstrate the noise. It does show that noise is compounded by cropping - if you look at the whole image (in post #7), the noise isn't nearly so bad, is it?
As Terry has demonstrated, it is possible to shoot at high ISO and get great results, but only if you know what you're doing and also to know when you're expecting too much/not going to succeed due to bad light (not just low light) combined with other factors. Also of course; how to make the most of it regardless - and not be too disappointed with the (to be expected) sub-optimal results.
With experience, you'll learn that in the scenario above, you would need to use positive Exposure Compensation to counter the effect of the mist on the camera's meter to achieve a good exposure to make the most of the light.
Also that high ISO is OK as long as you;
do not under expose AND
you will need to be close enough to subject so that cropping the images to achieve a good composition is not necessary AND
you shoot RAW AND
you know how to Post Process (PP)
In this shooting case, in PP, the contrast range needs expanding to bring the black point down (using a Dehaze feature should work well) and it also needs some decent Noise Reduction, plus the usual workflow.
However, all that said, in the precise shooting situation you faced above, another two stops exposure would have meant a shutter speed so low that the shot would almost certainly have been even more spoiled (blurred) by camera shake and/or subject movement. Your best bet might have been to get in to a better/closer position to shoot from as the light level increased.
Don't be discouraged, we all learn far more from our mistakes, so this was a valuable learning experience - and I commend you for getting up and out so early, that shows dedication.
PP of course, is another whole area of required expertise if you have any desire to become competent and achieve great results.
As are the skills necessary to stalk deer successfully, even in (my guess) Richmond Park - not to mention "luck" - although that usually improves with experience
It's a long journey, you'll never stop learning, but it can be fun.
All the best, Dave