That seems quite risky for them to not let your GP know you're pregnant when you're as far along as 14 weeks!
I can understand the dilemma OP, having been through 5 years of fertility treatments myself, I totally understand waiting list anxiety!
Is your appointment actually booked in, and how far along will you be by then?
I think it would be very difficult to actually have the fertility appointment whilst you are pregnant, because you would basically have to lie and pretend that you have been unable to fall pregnant... they will then start investigations etc as if you were infertile... but you're not. And the thing is, as soon as they do the blood tests and start looking at your hormones, and do a pelvic ultrasound, they will know you're pregnant anyway.
Personally I think I'd struggle to do all that with a clear conscious, so if the appointment is coming up soon, I'd be tempted to just delay it for a few weeks, say I was out of the country etc.
Once you get to the 8-10 week mark in your pregnancy it would be risky for your GP not to know, and also by then, the risk of miscarriage is vastly reduced. You will also need to see a midwife (presuming you are going to have NHS care).
Finally have you considered that if you are in some kind of accident or medical emergency, the fact that you are pregnant wouldn't be on your medical record, so they may give you drugs/ treatments etc that are not suitable and could harm you/ baby? It's worth thinking about - you need to weigh up these things.
I don't envy your position though. I think if you're quite early and your appointment isn't for a while, I'd be tempted to keep it... but not beyond 8-10 weeks. By then you will really need to get things moving with your maternity care.