Pathway is absolutely right and has laid the various roles out very nicely. In terms of my own quick summary I would say;
Counsellor - Talking based therapy for mild mental health issues with middle class patients who can afford regular sessions.Typically counselling isn't offered on the NHS so only those willing to pay privately will benefit.
Psychiatrist - Head of the mental health team. Medical doctor. Diagnose, treat (with drugs and ECT in rare cases) and generally lead mental health services often with management roles. They also have a responsibility to look out for the patient's physical health and exclude organic causes of mental illness.
Clinical psychologists - Work alongside the psychiatrists/nurses/social workers if your MDT is lucky enough to have the budget for one. They will deliver therapies and undertake a lot of assessment (e.g. cognitive testing). They also have roles in education and research as part of their work.
Psychotherapy can basically be undertaken by anyone but it is typically the reserve (or at least should be) of clinical/forensic psychologists and psychiatrists. Psychiatrists are all trained in psychotherapy but rarely deliver it day to day.
As mentioned counselling isn't a protected title and it is largely a "talking based therapeutic approach". It is an extremely saturated market with a varying quality of practitioners. It is very difficult to get a good client base going, at least one large enough to make a full time job with a decent wage out of it.
Psychology is also a very saturated and competitive market and I wouldn't advise pursuing it, especially not as a mature student. It will likely take at least 10-12 years to become fully qualified and even then many people never get on the training doctorates due to the competition. Jobs on the other end are becoming increasingly under pressured too what with NHS cutbacks. There are diminishing salaries and a diminishing number of jobs.
Psychiatry is in the best shape job wise at the moment and they are literally crying out for medical graduates to train in psychiatry (it's an unpopular career choice for medics). Psychiatrists as the head of the mental health team hold a very protected position when it comes to cuts etc. But obviously as it requires a medical background it isn't a viable option for many people interested in this field.
In terms of a rough salary guide I would say;
Psychiatry (consultant level) - £75,000-£120,000
Clinical psychology et al - £45,000-£65,000
Counselling - £0-£30,000 (I imagine it varies massively though)