I'll do my best to answer your questions. As I said, I did the Hull course, but I supervise trainees on a regular basis now and we have some assistant psychologists in our service who are usually trying to get onto DClinPsy training.
"I realise that I might be excessive in presuming that obtaining a PHD is more difficult than it is, but I'm having a hard time believing everyone's laid back attitude towards it. I mean if you have 150-200 applicants applying for a class that accepts about 10 students, I'm not sure how you can't be concerned about that." - the problem that everyone is picking up on is that if you ONLY focus on your studies, to the exclusion of all else, you are going to burn yourself out. No ifs, not buts, it will happen. That is why people are giving you the advice to chill a bit.
"I've never got an A grade in my GCSEs, I actually got a D in A-level psychology, I couldn't get a first class grade last year, why should I relax my attitude on the difficulties of getting a PHD?"- this is new information and, honestly, will make it slightly more difficult for you. But that doesn't mean that burning yourself out will work either. Part of the interview process for Clin Psych will ask you about how you look after yourself and you need to start getting serious about that self-care in order to prevent yourself from burning out.
"If clinical psychologists do a lot more than therapy, can you tell me what it involves? I've been trying to research what exactly a clinical psychologist does, and all the official career advice websites just say they design and deliver treatment plans for people with severe disorders." Okay, we do so much more than that. For example, I work in physical health. I work with people with chronic pain. Part of my job is individual therapy. Part is joint work with our physiotherapists, and also delivering group-based treatments for patients. I also do research, supervise trainees and other members of staff, liaise with other professionals in the hospital to develop services (involves lots of audits/service evaluations, pilot projects, etc - part of your job is expanding services whether you like it or not!). I also do consultation onto wards where there are distressed patients, and I work with staff teams (sometimes not even directly with patients) to give advice. I teach on local courses, and sometimes talk at conferences about our work. I work in the NHS. The NHS expects more from its clinical psychologists than just therapy - that's what they have IAPT workers for. Clin psychs are critical thinkers, we don't just deliver therapy, we question services and practice and try to make it work for more people, and work better (ymmv in other countries, I'm speaking for the UK here).
"I'm really considering making it an actual goal to eventually work in Grendon. I'd have to do some career research however on what type of psychologists do this, whether it's standard psychotherapists, or clinical psychologists or forensic psychologists or all of them." Clinical psychologists work in forensic settings in the type of work you're talking about, but so do some forensic psychologists. Clin psychs also work in child psychology, learning disabilities, older adults, physical health, and mental health, amongst other areas. We're everywhere!
"I have no idea whether I'd like research to be a part of my job, literally no idea, but I have a growing interest in learning from the mountain of academic literature about the different subfields of psychology and how integrating that knowledge can aid me as a clinical psychologist. So that might be a tell-tale sign." You don't necessarily have to do formal research, but you are going to have to do audit and service evaluation. They're paying you more to do those things - they're part of the "added extra" that you're bringing to the team, in terms of critical thinking skills.
"If all I need is a BA and lots of work experience to apply for a doctorate then what extra benefit would a masters give me? Do the benefits outweigh the student loan costs?" Clinical psychology is a scientific discipline. To get on a course, you'll need to demonstrate an ability to critically appraise information, and to be able to explain clearly what you think will make you a good clinical psychologist. It's the added skills that you will have that make you attractive, and you can get lots of these from a masters - honestly, I'd be thinking about going to a brick university for your masters - OU can be quite isolating I would imagine. You'll also need work experience and the best experience you can get would be an assistant psychologist post. Applying for those, particularly if you get through to interview, will show you the areas that you might be lacking in and give you ideas as to how to bring the areas you lack up to speed with the areas you're really good at.
Sorry it's taken me a while to reply, I'm not here very often! Good luck with it all :)