Hi. As well as being a qualified CBT therapist I hold counselling skills level 3 and I'm a registered social worker. It depends on what type of therapist you want to become.
If you want to become a couples therapist you'll need to qualify as a counsellor first, and then do additional training in this. Relate offer the training. You can often do the first few levels of Counselling Skills as a night class, then when you reach level four and five you'll need to get a placement in a service I believe.
I'm a CBT therapist, it's quite a long pathway. You can either do a psychology degree then train as a Psychological Wellbeing Practitioner via the NHS and then step up a few years later into CBT training, which is mostly funded by the NHS placement you're in (you apply for a trainee PWP and then trainee CBT post both of which pay you while you train), or a core profession (nursing, social work, occupational therapist) and then CBT training via the NHS once you have enough experience. Or you can pay privately but you must ensure the course you access is properly accredited so you can register with the BABCP at the end.
As a private counsellor, once you're qualified, you can charge £50-80 per hour depending on area, but bear in mind that you're not guaranteed clients and this of course doesn't cover time spent preparing for sessions, writing notes and researching. As an NHS therapist counsellors get paid on a band 5 salary, CBT therapists a band 7.
Is there a reason you want specifically to work with couples and families? What's your current experience in the area/qualifications?
Here's some info on becoming a counsellor: https://www.bacp.co.uk/careers/careers-in-counselling/training/
If you haven't already done any relevant voluntary work I highly recommend that to ensure it's something you'd actually enjoy doing for a career. I volunteered for the Samaritans for a decade and it was invaluable, both in confirming what I wanted to do longterm (I'm in my thirties now) and in giving me myriad skills to draw upon in the job/job interviews.
There are loads of 'courses' online you can do called 'diplomas' that are literally meaningless so watch out for those. I once went to a counsellor who claimed they were qualified in CBT. As a CBT therapist I was a bit confused when at the first session she gave me a self help book to go through by myself. I checked the register, nada. Turns out she'd done a two day 'course' on CBT and was now calling herself CBT qualified. Don't waste your money.
You could also consider contacting some local private counsellors/CBT therapists/couples therapists and asking if they're open to providing some info on pathways into the career, by paying for a session. I'm always happy to coach people on how to get into this role as it can be quite a convoluted and long journey and it's not always easy to know how to get there unless you know someone who does it! I don't work privately though.