That was pretty kind of them to let you know the facts, but reading between the lines it says "we're obligated to help you, but in reality, we can't.".
Remember the NHS policy is you must be seen within 18 weeks (right?). You are completely free to demand a referral to a different GIC. The waiting times at other GICs are available at:
http://uktrans.info/waitingtimesSo I would pick a different GIC and take your letter back to your GP, asking for a new referral. That's what I did when I was originally referred to London (which had a 1 year waiting time), and I was seen at my new GIC choice within 2 months.