I'm a counsellor with a diploma. My training took four years part time, although it was full on with mentoring then counselling placements, weekly personal development groups over three years, about 5 5000 word essays, 60+ hours of personal therapy, observations and a research project as well.
I have twice-weekly professional supervision and I undertake around 50 hours of CPD (training workshops) a year including.
As a single parent and carer with a disability, I couldn't afford the master's degree qualifications. I did train at the same time as a very old friend who did a master's and our reading lists for our courses were almost identical — her training involved far fewer essays (a lot less assessment on theory than my diploma involved, just reflective practice essays, which mine had also); hers did have a research thesis rather than a project, though that can be left out and the qualification then becomes a pgdip.
In terms of academic ability, I do have two previous master's degrees in different subjects and a degree from Oxford.
The master's degree qualifications and the level 4/5 diplomas aren't very different other than the details above.
A PhD, of course, is a different matter entirely. It does involve a lot more theory, but similar skills practice and personal development from what I've seen in prospectuses.
If you search on directories, you can usually see where someone has a PhD, as therapists with those will usually advertise them there. Also, a psychoanalyst will have many years of intensive training, so might be more what you're looking for.
For people worried that counsellors don't have much supervision/regulation, check they're with an accredited body: most directories only allow registered therapists who have monthly supervision and insurance to advertise.