Never easy to prioritise when one has so many diverse treatments to cover, others that could also fall in the same category, weight-loss for example, aspects of cosmetic surgery, etc. Never mind the patients and their age and sociometrics..... Having studied some aspects of public policy making, I for one would not want the job as, by definition, it will always be a judgemental decision.
As to the question here, one word of caution and that would be one has to be very careful about definitions of transgender and transsexual - throw in crossdressing too and, in practice, there is a continuum and not separate categories. However, I agree with the comment that the mental and other consequences of not assisting transsexuals, (and especially when they are young as that is more cost effective), well then the other costs on the NHS may well be higher. The NHS require an awful lot of commitment and psychiatric assessment before any operation is undertaken....at least two years full time living and a minimum of two independent psychiatrists reports over the period too, so the decision is not a lightweight one.
In practice, a lot head overseas, not because of the wait or cost, but the best surgeons tend to be offshore and the candidate will save to take advantage of this.