You're entitled to ask for a referral for specialist treatment on the NHS. However, whether you'll get the referral depends on what your GP feels is clinically necessary in your case.
Your GP also generally understands your health history and treatments better than anyone else and will base any decision for a specialist referral on this knowledge.
If you ask your GP to refer you to a specialist, they'll probably suggest that you first try various tests or treatment options to see whether your condition improves.
For more information on NHS referrals, visit: NHS - Referrals
Nottingham University Hospitals (NUH)
If you have been referred to NUH for treatment and would like details on estimated waiting times, letters you should have received or general health advice relating to the referral, visit: NUH NHS Trust - Referrals