Usually every referal starts with your GP. If you have a GP who's good and listens to you, they should be able to refer you to any NHS hospital you choose. Very occaisionally there may be an issue with NHS funding if you want to go to a hospital outside of your area, but if the GP believes there's a good reason for it (e.g. severity of symptoms or a need for a particular specialist) there shouldn't be a problem. And you're always entitled to at least two referals, so if you're not happy after one appointment, you're entitled to a second opinion.
The only problem that sometimes occurs is that you may be being referred to a particular consultant, but when you get there you find it's a different person you actually see. Sometimes appointment letters say - "You will be seen by [consultant's name] or a member of his/her team." If you find you're seen by a member of the team instead, you don't get much choice in the matter unfortunately.