you get a referral from your GP -and if you don't want to wait for months ask him if you can go privately. This link will give you some sound advice.
http://www.bdadyslexia.org.uk/
if it for a child I would urge you to get the child tested asap as the younger the child is when diagnosed, the better they cope.
Some schools test for dyslexia. Grand daughter slightly dyslexic, left-handed, reads voraciously, spelling wobbly, dances like a dream. Not always an insurmountable problem.
@daisy -absolutely not an insurmountable problem - relatively easy to put together coping strategies but if left a long time the child can develop severe behavioural problems which can often be misdiagnosed in the school environment. My youngest daughter was diagnosed at 8 years old and went on to gain 8 A* GCSE's -after being told (us) by her stupid headmaster in infant school we were expecting too much of her and she was obviously going to be a low achiever.....