If you're aiming for competitive conversion courses - and bear in mind you don't necessarily need them - then I would say you probably need to think in terms of going into the conversion oven-ready for the doctorate itself in terms of experience. They may want people who are going to boost their conversion figures for the clinical and educational doctorates rather than just using them as a cash cow. Once you're on those courses, you'll probably find they're pretty full on as well. Having started full time on a competitive conversion course with professional doctorate links, I found that I wasn't going to be able to support myself and be a high achiever at the same time. I dropped down to part time pretty quickly and was able to turn out those precious distinctions and high merits whilst working. You can't count on continuing to hunt out experience whilst you're doing the conversion full time. You want to be able to concentrate on the work when it's needed.
Oven ready for the doctorate means at least 12 months full time paid clinical experience, ideally NHS, preferably within a mental health setting with CP supervision - assuming you're going for the clinical doctorate. Educational settings such as colleges for adults with learning disabilities are also good and they also would count toward the educational doctorate if you were interested. You want to try and get experience within two or more client groups. Volunteering is well and good but it often doesn't really contribute toward the core experience needed. It's slightly different for psychology grads who are free to offer themselves to be exploited as honorary assistant psychologists (read unpaid labour). If you're a non-psychology grad heading toward the doctorate, you want to be thinking about ways of gaining experience with different client groups, gaining core clinical skills - think roles such as therapy assistant for the allied health professions - and proving that you have the resilience to work within this clinical area.
If you've not got much clinical experience already, you need to hear this: you don't actually want to be a psychologist yet. You like the idea of it. Maybe you like the salary and the prestige the job title attaches. Maybe you like the idea of being paid a band 6 salary to train. Maybe you like the idea of counselling people and helping them navigate their troubles. Maybe you like the idea of what you think psychology is i.e. Freud, the Stanford Prison experiment etc. Until you've wrestled with computational models of language, spent four hours decoding a single paragraph of the first of thirty journal articles you've downloaded to read, grappled with SPSS...you don't really know the subject. Until you've spent time on a brain injury unit, in a secure forensic setting, in a stroke unit, in a paediatric assessment clinic, had to deal with staffing shortages which mean that people can't access therapy because they're waiting to be taken to the toilet for an hour and a half and then have to wait another half hour to be showered, washed and dressed and by which time they're in a foul mood and they have no interest in doing an assessment or therapy and they just want to swear and threaten people...you don't really know what you're getting into.
You're about three years at least from being ready. I think you need to get your feet wet first, maybe try a single module of psychology from an online course such as the University of Derby or through the OU, or through a MOOC. Take a job as a care assistant or a support worker if you can. But don't be jumping in headfirst. Give yourself options and build in exit routes. Using Master's funding on a conversion is a pretty big risk if you don't know what you're getting yourself into. I got halfway through mine before realising I was heading into a career cul-de-sac. Plenty of other people on mine pushed through and then shook off their psychology bug and went on to do completely different things because they realised they hadn't thought it through. Some people quit after failing the first essay and realising their idea of psychology and the reality were just too distant. Maybe 3 or 4 out of 30 eventually went on do psychology as a career and only about half of them as practising psychologists. I look back at it from my current role and I realise just how naive most of us were to believe we had a shot.
Consider alternative career paths - don't be blinded by prestige. If the job you would do if you found out you couldn't be a psychologist has nothing to do with healthcare or education or forensics - the core psychology areas - then you probably have no real interest in being a psychologist.