Flexible combined honours, what is it and where do they do it??

Each Uni that has this sort of degree program will do it slightly differently - and may call it something different.
Here are some examples :
Combined degrees - Undergraduate - University of Liverpool
Liberal Arts BA | University of Leeds
Arts and Sciences - Undergraduate Degree - Y000 - University of Birmingham
Combined Honours BA | Undergraduate | Newcastle University (ncl.ac.uk)
Natural Sciences - Lancaster University

There are a couple other unis where you can pick and choose which subjects you want to combine when you start. The FCH term is specific to Exeter, others just call them combined honours or joint honours degrees but often they are presented in a table format otherwise.
You may also want to consider courses in e.g. liberal arts, arts and sciences, or natural sciences depending on your preferred combination.
Note that just because in principle you can combine two subjects through FCH doesn't mean in practice it will always work - you basically have to pick modules subject to timetabling (the timetabling doesn't work around you as the FCH student) and so often certain combinations may not work, or will end up heavily constraining your options within thay combination.
I suspect this is part of why most joint honours courses are predefined combinations (also to ensure a level of intellectual coherence).
What subjects exactly are you looking at combining?

Wait is it just basically another version of a liberal arts degree? If so I’m not wanting to do it lol that’ll render my prospects useless.

Who has told you that - it simply isnt true.

I know a lib arts degree would be helpful for developing some skills but I guess I assume that if I want to get into a specific line of work something that’s less eclectic will be more beneficial eg. It won’t help with wanting to become a psychotherapist or whatever. A lot of people say that lib arts degrees have that downside and it’s a rich kids course.