This is not as Shaker says a black and white issue. Whilst undoubtedly more should be spent on Mental Health issues - we have unfortunately, in the case of Mental Health issues, got an outcome driven health service.
If you take for example Cardiac health - this has been transformed over the past 20 years by various interventions , drugs, operations. Perhaps the most impressive of these has been the advent of stenting as an almost regular procedure following MI's or indeed prior to expected cardiac issues in a patient.
These are easily measured outcomes - it is very easy to prove lives have been saved, improved and in some cases transformed by the various techniques available.
With Mental Health issues - there is no such clarity available. You are dealing with a much more difficult set of criteria to measure against - so the Health Service and also us as a society (it is worth remembering that the NHS and politicians will respond to what they perceive as the publics priorities are in these matters) rightly or wrongly probably channel too little money to mental health services.
How you begin to shift the funding is a tougher question - and one I don't feel upto even beginning to answer.