Fields of computer science with good academic outlook I gather that the academic job market is very tough for theoretical computer science PhDs with a low chance of obtaining a professorship.

Some things I have read with regards to this:
https://www.quora.com/What-is-the-academic-job-market-going-to-be-like-for-computational-complexity-theory-after-5-6-years
https://www.reddit.com/r/compsci/comments/njdyq/so_you_want_a_phd_in_theoretical_computer_science/
https://www.quora.com/I-want-to-pursue-PhD-in-Theoretical-Computer-Science-but-I-am-concerned-with-the-job-options-What-should-I-do

If this truly is the case, I would hope it is completely reasonable to ask what fields in computer science are looking a lot better in terms of their academic job market?

My background: I am an undergraduate and have been exploring my research interests since I aspire to become a researcher in academia. I am reading a joint degree in mathematics and computer science in the UK. Naturally, TCS was one of the first fields I started looking at.
What is hot right now doesn't actually concern you and the job market is volatile enough that in six or seven years when you enter the market it will probably be completely different. It isn't a great life choice to make decisions based on such changeable factors. 

You are better advised to study something that deeply interests you so that you are more likely to become hugely successful at it and (a) hope that the market isn't terrible when you graduate and (b) be assured that it will improve in time in any case. 

You also need to keep a bit of flexibility in mind as you study so that you can change with the times if necessary without losing your interest in things. 

So, do what you like to do and do it well. Doing what you think will give you some 'advantage' may turn out to disappoint you in too many ways to count.