Of course there are reasons why people could be refused: there are medical reasons, there are psychological reasons, there are personal reasons, there are social reasons, there are financial reasons... the question is: do any of them apply in your case? Without knowing the specifics, it's hard for us to advise you.
You said in your introductory post that you already live full-time as a female. That's a great step, and one that the Gender Identity Clinics (GICs) look on positively. There are several GICs in the country (and only one that treats under-18s) and as with everything else in the NHS they each tend to do their own thing, but as a general rule the following will help you:
- It'll strengthen your case if you can show your GIC evidence in writing that you've been living full-time in your preferred gender. This could include having changed your name; using your new name at school, uni, college or work; dressing full-time as a woman etc. My GIC wanted to see my Deed Poll to see my change of name, and wanted to know whether I was out to my family and at work (I was and am).
- If you're generally healthy, that will go in your favour. If you have any mental or physical health issues, it'd help if you take steps to get those seen to.
- Please bear in mind that most GICs have long waiting lists, and that you probably won't get hormones for quite a few months, or even longer. GICs generally like to see you for several appointments before prescribing hormones (although they can make exceptions) but many of us have waited months or even years before starting hormones. Please be patient and keep your expectations realistic.
- Hormones are neither here nor there financially, if you're on the NHS. At worst you can expect the standard NHS prescription charge for each item prescribed, but if you're on benefits they're free of charge. So they don't generally break the bank.
- Surgery, too, is free of charge... but think about transport to & from hospital, how long you'll need to recover (this depends on the procedure), who will take care of you whilst you recover, how you will support yourself (you won't be able to work for part of your recovery period).
- Your GIC will need evidence (your word is often enough) of persistent Gender Dysphoria. They'll ask you about your history and will need to see a pattern of dysphoria over quite some time. So be honest and tell them your history.
- Your GIC will need to see that your desire to live as a woman is due to your identity, and not due to a fetish. They'll assess you to make sure you genuinely have Gender Dysphoria. They're very experienced in this, so genuine cases don't tend to be refused.
If you haven't yet done so, the first step is to go to your GP and ask to be referred to a GIC. There are several, and they have different waiting times. So it all depends on which part of England you live in, really. The GIC will assess you and will recommend hormones and refer you for surgery, if appropriate.
Alternatively, if you're in a rush to get on hormones and don't want to wait for the NHS to do its thing, there are a couple of private clinics in London that could see you much sooner. They tend to cost something in the region of £200 per appointment.