IIRC you need to do the general physio bit first before specialising, but that is certainly a growth area. I wouldn't rush into setting up on your own, though. What a lot of physios do is to be self-employed within shared premises, each contributing to the rent/receptionist/website etc. And I can tell you from 35 years of self-employment that it is not stress free!
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jazzkat. Is it possible to identify the parts of your present job that you don't like and see if they can be changed before you jump ship? And ASC's Top Tip #728 - most organisations are under pressure to reduce headcount these days, so don't overlook negotiating voluntary redundancy.
[If it's a Quango, just commit an act of staggering but not criminal incompetence and be paid off with a non-disclosure agreement.]